We haven't been able to reproduce this bug I'm afraid as it is a very rare one. Since you have already done it two times in a short time span is quite amazing and if you find a way to reliably reproduce it we would love to hear about it.
I encountered this bug after the following chain of events:
1) The Pope in Rome was attacked by an adventurer (the adventurer's portrait looked Norse)
2) I offered to join the Pope's war against the adventurer
3) I had retinues and mercenaries already raised prior to the war
4) The Pope won all battles, my retinues and mercenaries did not fight
5) The war ended with the Pope winning
6) I left my retinues and mercenaries standing, and suddenly everyone was hostile to everyone