Perhaps a disgruntled customer? lost too many bitcoins, and wants revenge. Maybe trying to make satoshi bleed a little by causing the site to spend btc (tx fees) to send them back? Perhaps the competition? (Is there any?)

Also it could be less nefarious, some one codes a martingale bot incorrectly and doesn't check on it for a while.  that's not likely after seeing how its still going for so long.