Anybody have any idea what this could be?
Quite possibly a gambling site (maybe an exchange?) that offers "instant" withdrawal... if you follow all the way back to the "start" you'll find a transfer from what is most likely cold storage into a bunch of 15 BTC outputs, like this:
https://www.blockchain.com/btc/tx/093abc15e60f811f147763f86c2e773d5037a0b7ac764451ec5536eed4029a5cThen each of those 15 BTC outputs is slowly 'ground down' as payments are sent out.
This may be a dumb question but why not do it in one big transaction to save on fees?
While it is true that they may save on fees, if a service offers "instant" withdrawal and as soon as a user clicks "withdraw", that transaction is not showing... users get VERY anxious. I've also seen some services that just simply didn't have the technical knowledge of how to implement batched withdrawals.