Happened to me a few times...
In the olden (pre dating sites) days I met a guy on a night out. Dated for about 3 weeks, spoke most days, then out of the blue he stopped calling. Left him a couple of messages, never heard from him again.
Another one was a guy I had known for a while as we all drank in the same pub. He'd always liked me but I was with someone else. About a year later bumped into him, we were both single, and he asked me out. Made all the arrangements (he was picking me up from mine) and he never turned up.
6 months later I heard from him, when he'd told me he was single he actually was only 'on a break' from his girlfriend. They split up for good a few weeks later but it took him months to pluck up the courage to contact me! In the end we went out for about 8-9 months. He was a
tit boyfriend though and I ended up dumping him by text
I met a guy on a dating site who was an odd one...we had 3 dates in about 10 days, he was super keen then I text him to arrange a date and he said he was 'busy'...and then stopped replying. A few months later heard from him again, same thing. All in all I think I saw him about 12 times over the course of a year with massive gaps in between.
I would agree usually it is because they are dating loads of different women at the same time; or sometimes because they already have a long term partner and are just looking for a bit of side fun, get caught out and go to ground.