Bargain hunting is my biggest trigger
If it's not sales season or trying to find some crazy good deal on second hand selling apps, I wouldn't shop for unnecessary stuff at all. I can sit for months on a decision to buy something and then forget about it, if it's full price. But if I see someone selling it for super cheap, it's instant "omg, this is my chance, it's now or never!"
The weird thing I knew this about myself for the longest time, but I was still convinced that it's actually fine to shop like that. I'm paying much less for stuff. That's good, right? And if I only pay a fraction for this item, then I could get another item (if I come across a good deal, of course). And another. I was even doing some rough girl math in my head. 70% off means it's ok to buy 3 dressed instead of 1.
Ugh.