This was discovered in the same editing session that gave rise to this thread and so may be connected:
http://www.steinberg.net/forum/viewtopic.php?f=19&t=9243&p=61966#p61853
Rolling an event along a lane by scrolling its start point in the info bar causes any event in the way to be pushed down to a new lane. This can lead to a lot of new lanes all of a sudden.
This seems to be Front/Back related. If you move the event to Back before scrolling, it moves round the events. Try it - it’s an almost amusing diversion!
I’m being to get a picture, between this thread and the one linked to above, that the Front/Back feature is being used in lane editing in such a way that it’s producing some unfortunate side-effects.