My almost 20 month old always seem to be the one playing up at the classes/ groups we go to!
First up he has always been a bit wary of other toddlers and if any of them push him slightly (to the point it wouldn't hurt) or an adult gets in his way he just shrieks and cries - completely over reacting (imho).
Secondly - in the past he used to just keep out of the way of the other toddlers and he still does hang back a bit (he gets this from dh who e.g. waits til the other passengers have got off the plane before standing up rather than rushing to get off!) but now if they have something he wants or are in his way he has started pinching. Luckily he is only managing to pinch their clothes (intentionally or otherwise) and hasn't got any skin.
I think that his nature (somewhat sensitive it seems) means situations most other toddlers take in their stride, upset him and now the frustration he feels is manifesting itself with the pinching.
Anyone got any tips as I don't think time out would work at all - he'd be really happy to leave the room as he seems to prefer hanging out in the corridor to being with the scrum (I know that at his age that he shouldn't be expected to actively play with other kids but maybe he should at least tolerate them!)