There are several causes for bread to collapse or cave in (sunken top):
FIRST CAUSES:
Too much yeast
Liquids are present
There is not enough flour
Temperature of the liquids is not correct
RECOMMENDATION:
When correct salt and sugar amounts are used, a small increase of these two
ingredients may be a remedy. Or, try reducing the water called for by 1/4
to 4 tablespoons.
SECOND CAUSES:
High humidity or high temperatures are present
RECOMMENDATION:
At such time, add 1 to 2 tablespoons flour or decrease the liquid. A
quick-rise yeast may have been used.
TIP:
Too much yeast will cause the loaf to have a course, open grain and a
flattened or sunken top.