There isn't any official explanation of what happened but the general consensus of opinion is that the ground became so waterlogged that the extra weight was more than the lock wall could support and it collapsed.
If we hadn't changed our plans we would probably have been in Aylesbury, trapped the wrong side of this and it isn't something that is gong to be fixed quickly. Worse still we could have been in that lock when it collapsed.
After Phil caught up with us and we had stocked up on everything we decided not to do battle with all the bank holiday weekend boaters so we are still in Berkhampsted and planning to move on Tuesday. With the Aylesbury Arm off the itinerary we will now head for Braunston, though we may explore the mile and a half of the Wendover Arm that is navigable on the way.