Human error .....ace isn't it?...Not!
If it's any consolation I've been pulling my hair out ( the bit that I have lol) this afternoon wondering why I've had seriously thin negs.
I even exposed a second film and opened a new bottle of developer believing the previous was exhausted ( quite possible as it is old (ish)
Still got thin negs!
Gone round in circles panicing that it's a camera issue but shutter works fine, aperture opens/ coloses correctly etc etc
Eventually figured it's a metering issue .......had the meter set completely wrong and ultimatley underexposed all 36 frames by 3 stops!!!
Talk about schoolboy error!
Anyway, glad to hear that you look to have no issues with your kit, even if you don't know for sure what caused the lines.
I suspect that you're somewhat relieved too!