Under what conditions does SQL Server encrypt tempdb? When does tempdb get encrypted with Transparent Data Encryption?  What configuration, etc, causes tempdb to be encrypted this way?
When you encrypt the first database in the instance, tempdb will also be encrypted.  As per this reference on TDE:


  Transparent Data Encryption and the tempdb System Database
  
  The tempdb system database will be encrypted if any other database on
  the instance of SQL Server is encrypted by using TDE.


This will be the case when you set ENCRYPTION ON on a database.