I know it probably doesnt help too much. But the next time it occurs. It sounds like it is a local process. if you can get access when it happens. remote onto the server and look via task manager and see if you can work out what it is and kill it that way. once you have killed it then, it should free up the other backup job.
if you kill the spid.
you can then run kill [spid] with statusonly
and that should give you an estimated time for how long the rollback will take, if it comes back with 0, that can mean it is dependent on a non-sql possibly an o/s process to complete.
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