SQL Server Sch-S locks on unrelated tables
Potential Causes

There are two likely reasons for this


There's a foreign key on the table
There's a trigger on the table


There are less likely reasons, too


Computed column based on a scalar function
Indexed view maintenance


There's also the possibility that the blocking is due to metadata changing, where modifications to system views can block each other.

Figuring It Out

You can use sp_WhoIsActive. 

If you run it like so: sp_WhoIsActive @get_locks = 1;, it should help you get to the root of your blocking issue.