This is normal behaviour , once you shrink tempdb , sql tries to find out which parts of the files are in use , after ignorning these parts it releases the remaining space back to OS. The size of the temp db after shrinking the becomes the initial size and overrides the values set up initially by the DBA, more of it is explained in the link below.

http://enabledbusinesssolutions.com/blog/the-tempdb-why-shrinking-doesnt-always-work/

There is nothing wrong with shrinking tempdb , its the user databases that usually cause problems , tempdb grows and shrink constantly and as such needs the occasionaly DBA intervention.

I would suggest you setup alerts for disk space monitoring ,and enabled fast file inititializaion also lookup trace flag 1117 and 1118

Good luck