After research I learned that this is caused by birds. So I made some bird guards out of 1x1 inch wire mesh in order to keep the birds from getting at the bee house, disturbing the nesting tubes and killing the bee larva. In this case, leaf cutter bees. There appears to be some bees still in some of the nesting tubes, so I'll see if they emerge in the warmer weather.
I built a new bee house for both early mason bees for apple tree pollination and leaf cutter bees for midsummer pollination. This new bee house also has a bird guard in place and a door in front to place purchased bee cocoons.
