Yeah usually Ebay will do this to meet quarterly sales goals. It used to be deals like $15 off $75+, $10 off $50 but then they did 20% off total no minimum, then 15% but sadly it looks like the new norm is 10% with the occasional 15% sale. It's usually a 24 hour sale and posted on the front page. Sometimes it leaks early and big sellers will get notified and jack their prices up beforehand but at least you know a sale is coming. From my experience though, it's random.