When under the PDT you are restricted with how many trades you can take, thus making sure you take very selective trades. But at the same time very stubborn when cutting loss because you don't want to lose your trade already.
When over the PDT you tend to take some boneheaded plays because you CAN and keep going, but slowly those losses on bonehead plays adds up. THEN when you get near 25k you are literally back against the wall doing your best not to fall under that. Thus trading small size for thinking you'll take small loss. But wins will be small wins. You get impatient and take MANY trades because you CAN. Thus brings you back into a vicious cycle of trying to grow your account.