What can I do to prevent big holes in the bottom of bread loaves?
When I had a similar result many years ago, I finally figured out that the problem was a combination of how I shaped the loaves and letting them over rise in the pan.  It also looks to me that these loaves were twisted when formed.  If so, perhaps try a different method of shaping them such as a braid, for a similar look.  Moreover, check your recipe for how much the dough should rise in the pan before going into the oven.  Some call for rising to the level of the bread pan, some call for an inch (rarely more) above the edge of the pan.  Finally, make sure your oven is fully preheated, especially if using a baking stone.