I have had this exact same problem probably three times in the last
year with this formula. I've never really laid it to rest but I too am
leaning towards the 'too much water' theory... though I've long
believed it may also have to do with not forming the final loaf
correctly such that you end up with a continuous air pocket on top
which just blows up beyond control once in the hot oven...

It would be good to hear the 'real reason' from an expert though.