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HertogJan: The lower the condition, the higher the chance it jams.
Not sure, but IIRC it also matters if the character is specialized in that weapon type or not. If not, higher chance to jam.
Trueviking: But has weapon jamming always been working like that, or does it also have to do with what difficulty you're playing on?

I'm doing Expert atm.

Or did the introduction of 1.13 make them set jam chance way higher just overall?

I might be speculating bit too much ;p
I don't know. It's been a very very long time since I last played without 1.13. I do remember I always had the habit of repairing items due to the gun jamming in JA. When I capture a sector of a town and I can train militia, training militia and repairing items are the 1st things I do.