I've been having occasional mild external hemorrhoids for a while now, and haven't figured out the cause yet. The first time I remember I got it when I was doing high level physical training during my sports career (e.g., after doing squats with a partner sitting on my shoulders), and associated that with overstrain. But then they started to reappear without me doing the same amount of training, once or twice a year. Usually, it's a small hemorrhoid, no bleeding, etc. I also have two enlarged veins on my calve muscles, which resemble varicose, but I haven't had pains from those, and they aren't progressing.
My mum has been suffering from mild hemorrhoids as well, so there is some family tendency there.