On the other hand, if they were over trimmed and you headed up, the increase in apparent wind could easily overpower the boat.
A beginner is reaching with the sheet still hard in. The air flow is turbulent, and there is little power in the sail. When he luffs the air flow smooths out, and the heel will increase abruptly, for the pressure on the sail has risen drastically. This often happens to beginners , who are taken by surprise that they capsize, and worse of all they rarely understand why.
Nice going Sabreman.