I seldom do the Civil War questlines as I found them rather boring, but this time I decided to join the Stormcloaks. After retrieving the Jagged Crown, I stopped by in Whiterun before deliviring it to Ulfric and suddenly Galmar Stone-Fist entered through the gates and started fighting all the guards, as if he decided to attack the city all by himself. Is this normal behavior or is it a glitch or a bug? I'm mostly using cosmetic mods so I don't think I have anything that should interfere with the quest.

I didn't see anything in the wiki about that as a known bug, but that doesn't mean it isn't. I'm pretty sure he should not be attacking all by himself. I don't do the CW much either and the last time sided with the Empire, so the Stormcloak side was 3 or so years ago... I just don't remember all the particulars.

Too much mead makes Galmar a little rowdy.

I got to reload from a save before joining the Stormcloaks anyway. I wonder if I ever have done the Stormcloak attack on Whiterun before - not that I can remember anyway. And I just hated it - it's my favorite town and my char says hello everyday to very same guards she killed now. So the Stormcloaks are not for me, regardless of Galmar being a bit unhinged.