TM REQUEST Rollback SQL Server 2012 Always on Availability Group
According to this SO thread Session hanging with TM REQUEST and plan handle 0x0000 it indicates transaction manager servicing a DTC request which isn't supported on AGs (because if a failover occurs at that time it can kill your database - as documented by Microsoft but without instructions to reproduce it so we can go into more detail). 

You can confirm this by opening the DTC manager program and seeing what the numbers look like.