I also haven't no idea!
maybe if you are in a Java Standard Edition environment you should set also the TransactionManager in environment and then handle the transaction boundary your self as explain in user guide (http://docs.jboss.org/jbpm/v5.1/userguide/ch07.html#d0e2820). Instead if you are in Java Enterprise Edition i haven't any suggestion... :(
I have used bitronix 1.3.3 in SE!
Of course your problem is related to transactions....