It's commonly called "flying crust." There's a lot of discussion about it in the dedicated bread forums, like The Fresh Loaf.
Your explanation is half right -- too much rising is part of the problem. However your analysis of the room's heat concentrated on top isn't right.
I'm tellin' ya, the solution -- at least in this case is better handling. The OP is totally degassing the bread (bad!), and breaking down the gluten chains that hold the structure together (worse!) through rough handling. Then she allows it to rise too much before it goes into the oven. This leads to an uneven distribution of sizes of gas-formed cells (larger ones on top because they have less to lift). The best solution is to handle the dough more gently in loaf formation which will leave cells more evenly distributed through the yeast; partially depress the top to get relatively smaller cells in the top (there's a reason for the traditional sandwich loaf shape -- and preventing flying crust is it); and not over inflating the dough outside the oven, i.e., allowing the proper amount of oven spring.
Another solution is to form the loaves, allow them to rise partially, then remove them from the pan, turn them over and repalce in the pan. More of a PITA than handling the dough properly to begin with. The partial push down along the long axis of the bread, creating the sandwich loaf shape, is additional insurance.
Believe it or don't,
PS. If you're at all serious about bread you ought to scope out The Fresh Loaf
(The Fresh Loaf | News & Information for Amateur Bakers and Artisan Bread Enthusiasts
) now and then. There's a truckload of really good bakers over there with a lot of expertise. It's nice to get some other viewpoints ... which is a segue to a caveat
. They have a sort of "house" take on how to bake artisanal breads involving autolysis, little to no kneading, etc., which is not only not the only way to bake, but not particularly mainstream either. Not to say that it isn't a great way to bake. Without going too far off on a tangent, you've got to allow for the institutional propaganda.