Subject Transaction in limbo

I have a database, which have had to instances where there have been Transactions in limbo.
I know how to fix this.

I am on the other hand unsure why they appear.
What can cause this?
What should/could I check, so I can eliminate this from occuring.

The database is a Firebird 2.5 running Classic Server.

We have some 120+ database accross some 10+ servers. This is the only one, where this is happening.