First test with all content DLC (sunset invasion, old gods etc.) + Unit Packs. Got past the normal crash time.
Second test without African, Byzantine and Celtic unit packs, but all other unit packs I own still active. Still works
Third test without above mentioned + Europa converter, customization pack and military orders unit pack. Still works
Fourth test without any unit packs, just content DLC. Still works
Fifth test again without any unit packs, just content DLC, this time making sure I do exactly the same stuff I did last time (namely win my war with pictland). Okay NOW it crashes
This implies it is indeed someone raising an army. I'm the only norse in the game, with the exception of a tiny count in the middle of norway. I don't have to actually win my war with pictland, destroying their army and then the army of Galloway seems to be enough.
Sixth test activating warriors of faith unit pack. (I only had 3 unit packs left, norse russian and warriors of faith). Crashes
Seventh test, deactivating warriors and activating norse again. Works
Okay so that was odd, definitely didn't work with norse activated before, but now it does. I'm going to run it without norse again up to the start of the fight in galloway, and then see if I can reproduce the crash within a few days of it normally happening.
Eighth test, deactivating norse and running until I find the crash. Save created within a day or two of when the crash should happen.
Ninth test, activated all DLC except norse and ran from crash test save, it didn't crash. Deactivating all DLC again and testing crash test save (probably should have done this first). Crashes exactly 1 day after the save.
10th test, loading from crash test again with just norse activated. Crashed on the 30th (save is 23rd I think)
11th test from crash test with just celts active. Works.
Save dump (should crash within a few days of loading)
http://www.filedropper.com/scandinaviacrashtest
tl;dr: I had inconsistencies with getting specific DLC to make it work. I mainly play in ireland, scotland and denmark, so it's quite possible it's celtic unit pack too. The last time it was crashing badly was when I was just finishing with ireland and declared war on all of england and scotland, but I don't think I have a relevant save to test what was going on there.