A transaction is left open obviously.

I don't know the application, but a good candidate is a timeout.

-The connection was running a long query

-It was timed out

-The connectoin was reused (or the code continued) without rolling back the open transactoin.

Developers sometimes forget about the fact that a timeout does not close the transactions.

If the issue occures regurarily, set up a trace with

-Attention

-SQL Batch completed

-LOGIN-Logout events

and watch...

(If you see sp_resetconection call, it can be handled as logout/login, it rolls back every trans)