Encounter deadlock even when there is no relation between tables
I could not understand the cause of the deadlock exactly because two
  DIFFERENT tables are involved and what I have read so far was when
  only one table is involved !!


Deadlocks occur when 2 or more sessions are waiting on each other. It doesn't matter if the deadlocked resources are rows in the same table, different tables, entire objects, etc. 

I ran your scripts to reproduce the deadlock, extracted the xml_deadlock_report column from the system_health XE trace, and saved the deadlock report XML to a file with an .xdl extension. Below is the graphical view from SSMS. 



As you can see, the SPID 58 victim was waiting on a Purchasing.PurchaseOrderDetail update keylock, blocked by the SPID 53 exclusive keylock on the same key. SPID 53 was waiting on an update keylock on Purchasing.PurchaseOrderHeader, blocked by the exclusive lock SPID 58 held on the same key.

Below is the raw xml_deadlock_report XML.

&lt;deadlock&gt;
  &lt;victim-list&gt;
    &lt;victimProcess id="process1a668c9eca8"/&gt;
  &lt;/victim-list&gt;
  &lt;process-list&gt;
    &lt;process id="process1a668c9eca8" taskpriority="0" logused="400" waitresource="KEY: 6:72057594048348160 (4ab5f0d47ad5)" waittime="1316" ownerId="2500187" transactionname="user_transaction" lasttranstarted="2018-07-24T06:02:13.303" XDES="0x1a85b00c490" lockMode="U" schedulerid="3" kpid="2932" status="suspended" spid="58" sbid="0" ecid="0" priority="0" trancount="2" lastbatchstarted="2018-07-24T06:02:31.437" lastbatchcompleted="2018-07-24T06:02:31.433" lastattention="2018-07-24T06:00:22.400" clientapp="Microsoft SQL Server Management Studio - Query" hostname="SQLSERVER" hostpid="15904" loginname="DOMAINNAME\USERNAME" isolationlevel="read committed (2)" xactid="2500187" currentdb="6" lockTimeout="4294967295" clientoption1="671090784" clientoption2="390200"&gt;
      &lt;executionStack&gt;
        &lt;frame procname="adhoc" line="1" stmtstart="64" stmtend="278" sqlhandle="0x02000000d0c7f31a30fb1ad425c34357fe8ef6326793e7aa0000000000000000000000000000000000000000"&gt;  unknown    &lt;/frame&gt;
        &lt;frame procname="adhoc" line="1" stmtstart="26" stmtend="326" sqlhandle="0x02000000b66e4532b6686980baf466a9597258f2464dd4f50000000000000000000000000000000000000000"&gt;  unknown    &lt;/frame&gt;
      &lt;/executionStack&gt;
      &lt;inputbuf&gt;               Update Purchasing.PurchaseOrderDetail               Set OrderQty = 2               where ProductID = 448 and                     PurchaseOrderID = 1255   &lt;/inputbuf&gt;
    &lt;/process&gt;
    &lt;process id="process1a66883d848" taskpriority="0" logused="9484" waitresource="KEY: 6:72057594048413696 (4bc08edebc6b)" waittime="16176" ownerId="2500193" transactionname="user_transaction" lasttranstarted="2018-07-24T06:02:16.577" XDES="0x1a85e004490" lockMode="U" schedulerid="5" kpid="20416" status="suspended" spid="53" sbid="0" ecid="0" priority="0" trancount="2" lastbatchstarted="2018-07-24T06:02:16.577" lastbatchcompleted="2018-07-24T06:02:16.570" lastattention="2018-07-24T06:01:28.820" clientapp="Microsoft SQL Server Management Studio - Query" hostname="SQLSERVER" hostpid="15904" loginname="DOMAINNAME\USERNAME" isolationlevel="read committed (2)" xactid="2500193" currentdb="6" lockTimeout="4294967295" clientoption1="673319008" clientoption2="390200"&gt;
      &lt;executionStack&gt;
        &lt;frame procname="AdventureWorks2014.Purchasing.uPurchaseOrderDetail" line="39" stmtstart="2732" stmtend="3830" sqlhandle="0x0300060004cc856a0ee00a016ba3000000000000000000000000000000000000000000000000000000000000"&gt;  UPDATE [Purchasing].[PurchaseOrderHeader]              SET [Purchasing].[PurchaseOrderHeader].[SubTotal] =                   (SELECT SUM([Purchasing].[PurchaseOrderDetail].[LineTotal])                      FROM [Purchasing].[PurchaseOrderDetail]                      WHERE [Purchasing].[PurchaseOrderHeader].[PurchaseOrderID]                           = [Purchasing].[PurchaseOrderDetail].[PurchaseOrderID])              WHERE [Purchasing].[PurchaseOrderHeader].[PurchaseOrderID]                   IN (SELECT inserted.[PurchaseOrderID] FROM inserted    &lt;/frame&gt;
        &lt;frame procname="adhoc" line="3" stmtstart="64" stmtend="278" sqlhandle="0x02000000d0c7f31a30fb1ad425c34357fe8ef6326793e7aa0000000000000000000000000000000000000000"&gt;  unknown    &lt;/frame&gt;
        &lt;frame procname="adhoc" line="3" stmtstart="28" stmtend="250" sqlhandle="0x02000000cac3a70ebbbc20e8360adb397f4986f2ced020140000000000000000000000000000000000000000"&gt;  unknown    &lt;/frame&gt;
      &lt;/executionStack&gt;
      &lt;inputbuf&gt;  Begin Tran    update Purchasing.PurchaseOrderDetail  Set OrderQty = 4  Where ProductID = 448 and        PurchaseOrderID = 1255   &lt;/inputbuf&gt;
    &lt;/process&gt;
  &lt;/process-list&gt;
  &lt;resource-list&gt;
    &lt;keylock hobtid="72057594048348160" dbid="6" objectname="AdventureWorks2014.Purchasing.PurchaseOrderDetail" indexname="PK_PurchaseOrderDetail_PurchaseOrderID_PurchaseOrderDetailID" id="lock1a7fea46a00" mode="X" associatedObjectId="72057594048348160"&gt;
      &lt;owner-list&gt;
        &lt;owner id="process1a66883d848" mode="X"/&gt;
      &lt;/owner-list&gt;
      &lt;waiter-list&gt;
        &lt;waiter id="process1a668c9eca8" mode="U" requestType="wait"/&gt;
      &lt;/waiter-list&gt;
    &lt;/keylock&gt;
    &lt;keylock hobtid="72057594048413696" dbid="6" objectname="AdventureWorks2014.Purchasing.PurchaseOrderHeader" indexname="PK_PurchaseOrderHeader_PurchaseOrderID" id="lock1a815dda900" mode="X" associatedObjectId="72057594048413696"&gt;
      &lt;owner-list&gt;
        &lt;owner id="process1a668c9eca8" mode="X"/&gt;
      &lt;/owner-list&gt;
      &lt;waiter-list&gt;
        &lt;waiter id="process1a66883d848" mode="U" requestType="wait"/&gt;
      &lt;/waiter-list&gt;
    &lt;/keylock&gt;
  &lt;/resource-list&gt;
&lt;/deadlock&gt;


The sample AdventureWorks database includes an UPDATE trigger on Purchasing.PurchaseOrderDetail table that contributed to this deadlock. The trigger inserts into Production.TransactionHistory and also updates both Purchasing.PurchaseOrderHeader and Purchasing.PurchaseOrderDetail whenever ProductID, OrderQty, or UnitPrice columns are updated.

In summary, the sequence leading to this deadlock was:

1) connection 1 updated a Purchasing.PurchaseOrderHeader but did not commit

2) connection 2 updated the Purchasing.PurchaseDetail table and the fired trigger was blocked when trying to update Purchasing.PurchaseOrderHeader 

3) connection 1 tried to update Purchasing.PurchaseDetail but was blocked by connection

4) SQL Server chose connection 1 as the deadlock victim, allowing connection 2 to autocommit