Usually, I mix the butter and sugar first, then add the banana mashed with a little milk, and I put dry ingredients in last. But this time I put the baking powder with the butter and sugar and egg mixture, and my bread split in half while baking. That had never happened before! Did putting the baking powder with the wet ingredients first mess it up?
First, baking powders are typically double-acting leaveners, where gas is created in two phases: (1) when mixed with wet ingredients, and (2) when heated. By you mixing the baking powder early, it had more time during this first phase, thus giving your bread more rise before it went in the oven.
Second, as @Joe described, the cracking results from the crust setting before the leavening is done. Since your bread had extra leavening on this batch, the amount of rise may not have been able to keep up with the timing of the crust setting.
If you decided that you want to continue mixing the baking powder early, you could try either: (1) covering the bread dish with foil for part of the baking time, which might help slow down the crust setting (since moisture released from the surface would be trapped and keep the crust soft), thus making it more forgiving to the extra leavening; or (2) you could reduce the amount of baking powder or just finish mixing it all very quickly.