I've not got huge experience - but I've never heard of ANY mobo of this type/size capsizing - it seems especially odd, given that it was on the river.
I wonder whether they were wearing lifejackets (I could envisage that being on the river, they may not have thought it necessary), and if they weren't, would it have made a difference?
Is there a marine equivalent of the Air Accident Investigation Unit that will reveal more on this over time?