My experience is that stops in the main are clustered just below...

My experience is that stops in the main are clustered just below the critical points like round numbers for example. Not much point putting stops right at support levels. So in the case of Alexium there would have been a lot of 50 cent round number stops clustered at 49.5 and 49 cents be they mechanical or notional stops IMO. When 50 cents was breached in the post 14:00 period then the sell volume surged again with 683K dropped in the next hour. The 60 minute period from 13:00 when 50 cents was hit but not breached accounted for only 234K and I think that supports the theory to some extent. In any case I imagine that the approaching 4C would have triggered the positioning of a lot of 'just in case' stops.

It is worth considering that when people set mechanical stops the brokers have access to this information so you might draw your own conclusions from that.