Cutworms or Cutter Beetles that hide in the soil immediately below the plant during the daytime. You can fool around and catch them with your hands and smash them after dark, or just before dawn, or use a pesticide on the soil.
Generally, such leaf damage in the end of August is not so harmful for deciduous bushes, soon they would loose their leaves anyway.
Derek Toub - I wonder if you realize you've shared far more posts than the one that relates to bougainvillea.
And, the beetles that go to sleep in your soil under the plant will be back next year, and the next year, and the next year... Tolerate them now and you're setting a situation which only gets worse.
Do you mean on Imgur? I’ve only ever put two posts on there (although it will show other people’s posts if you scroll down). Or did I accidentally post this question twice?