OK, clarification may be needed here. Technically, my bees didn't split. I split them on purpose. I had one double deep hive that seemed to be overflowing with bees. Every evening there were lots of bees on the outside of the hive.
This bee "bearding" happened to me last year when it got really hot and humid about this same time of year. There are too many bees for the hive and there is too much heat inside, so they go outside to cool down.
Since I lost a hive last year, I wanted to be more prepared this year with a few more hives just in case another string of bad luck came my way. So the decision was made to split the hive. I haven't done this myself before, so it was going to be interesting.