Urgent help with TempDB log file growth mystery
What you are you seeing is by design for tempdb Transaction Log file. I wish Microsoft had better documentation on this.

How is the transaction log growth increasing upto 80% and then
lowering down back to 0 and then increasing and growing and so on ,
with reaching almost max 90-95%.

From the First reference article:

Tempdb is not recovered in the event of a crash, and so there is no
need to force dirty tempdb pages to disk, except in the case where the
lazywriter process (part of the buffer pool) has to make space for
pages from other databases.
A checkpoint is only done for tempdb when the tempdb log file reaches
70% full – this is to prevent the tempdb log from growing if at all possible (note that a long-running transaction can still
essentially hold the log hostage and prevent it from clearing, just
like in a user database).
Of course, when you issue a manual CHECKPOINT, all the dirty pages
are flushed, but for automatic checkpoints, they’re not.

I suggest you monitor your tempdb Transaction Log usage and set it to a reasonable size that under normal workload it does not have to grow and shrink.
I do not know exactly why your tempdb logfile filled to 100%. There has to be a reason why the log file did not get truncated.
Will indirect checkpoint help?
In most cases no.

Any non-logged “bulk” operation that qualifies for an “eager write” in
tempdb is not a candidate to be flushed by the recovery writer (the
internal thread that runs the indirect checkpoint).
This raises an important question: which data load operation is
minimally logged on tempdb? This is important to know because
minimally logged operations on tempdb will not be flushed by the
indirect checkpoint. The following list can be used to assist you in
understanding which load operations on tempdb will be minimally logged
and which will not.

For details read Tempdb – Here’s a Problem You Didn’t Know You Had by Fabiano Amorim
I noticed this question is for SQL Server 2014, if someone is reading this for SQL Server 2016 read the below documentation by Microsoft.

Database Checkpoints (SQL Server) - check the &quot;Indirect Checkpoint&quot; section.
Change the Target Recovery Time of a Database (SQL Server).
Indirect Checkpoint and tempdb – the good, the bad and the non-yielding scheduler by Parikshit Savjani

Reference:

What does checkpoint do for tempdb? by Paul Randal
See question and answer section Inside the Storage Engine: What’s in the buffer pool? by Paul Randal