The longest you should ever soak a head is about an hour in my opinion. Anything more is over kill and it can cause issues like this. All a head soak does is allow solution to wick up into the dry ink that is causing the clog. After an hour, the solution has already done it's job as well as it can. As stated above, what happened is your pump didn't hold the solution in the cap perfectly. So a little solution started to slip down the tube causing a siphon. Once that starts, it doesn't stop until the cartridges are empty.