I have to agree with that, it seems to always happen when there is a big noisy group / running / bumping and some other sort of commotion. And yes, they are quite able to get stuff out of front pockets, as by now most tourists know (one would hope) not to carry wallets in their back pockets.
I got hit by the worst group of pickpockets ever on the Barcelona metro who attempted just this tactic. With carry-on bags in each hand, I wasn't moving quickly and didn't really think that the group of people crowding the door (with me between them and the door) might be attempting to steal something. They were just.... really really bad at it. The guy going for my wallet was so off-target that for half of it I actually thought he was trying to feel me up, rather than steal anything. End result, when the train stopped one of them made a half-hearted attempt to carry off the bag I had a firm grip on, so he failed pretty miserably. The rest of them got nothing.
Based on actual crime witnessed, I'd rank Madrid higher, but that's probably due to total time spent in each city.