We have had this problem, and we decided it was from people over tightening the lids. If folks crank on the lid it will stretch the O ring a lot faster. Constant use will do it as well. Since the lids cost about $8, it's not a big deal to buy a new lid, but having it fall off into the sand on a trip sucks.
"We are all visitors to this time, this place. We are just passing through. Our purpose here is to observe, to learn, to grow, to love....and then we return home."
Australian Aboriginal Proverb