SQL Server TempDB transaction log Issue
TempDB is no different than any other database.  All actions must be logged, and log space will only be reused once the log records for a given transaction have either been committed or rolled back.  

As @KrisGruttemeyer said in a comment, you can find out what type of action is causing the transaction log to grow by looking at:

select d.name, d.log_reuse_wait_desc 
from sys.databases d 
where d.name = 'tempdb';


This will likely show you either 'active transaction' or 'nothing' for tempdb.  If you see active transaction you know some transaction is open and has not been committed yet.  You can see what transactions are open by looking at various system DMVs including, sys.dm_tran_active_transactions, among others.  

For a much more in-depth look at what is causing your tempdb to grow, check out this question.