Unfortunately its a old craze, 2 guys on a moped chased me down a cycleway a few years ago, kicked me of. Broken hand and wrist. Posted the incident on here. There was no attempt to rob me etc, it was just a laugh to them.
Police had no evidence ( now I have cameras running full time fore and aft) but had a fair idea who it was. Now if a moped looks like its up to no good I slam the brakes on and dismount. Also a guy on my commute last year got surrounded by 4 or 5 of them. He stopped as he felt it was safer to fight on his feet, luckily they gave up as he looked like he might be too much trouble for them.
There's just no respect and a cyclist to them is just some weirdo who's a easy target. I have no clue as to the percentage that get away with this kind of attack but I suspect the danger of getting caught to them is negligible so well worth the giggle they get.