OK, this is going to get me in trouble, but when I'm jammed in the doorway of a train with my knees against someone's folder, the train pulls into the final station and I realise that the owner of the bike has been sitting in comfort while I've been bumping against his bike, I get annoyed. I think it's pretty selfish that I have had to deal with his bike on my journey and he hasn't. And I'm a folder owner myself - no jealousy involved.
Of course, I don't vandalise it and I'm not condoning this action. My personal feeling is that a bike is inconveniencing someone on the train - and if there are people standing, it is - then one of those people should be the bike owner. My train leg is short (~30 minutes if I catch the all-stations), so I only take a seat if there is room near the seat for the bike. TBH this is only partly for other commuters; I'd also hate to get off the train only to find my folder got off at an earlier stop!
The person who vandalised your bike was probably just a random thug, but it may have been someone having a bad morning who felt aggrieved by your possessions in the little room he had to stand.