I found myself breaking them with my finger nails. While reaching for his testies my fingers would snag themselves on the ringlet of the condom, two secs later the condoms split. It still happens from time to time when I don't recall not to go for his testies the condom snags and splits.
But using lube any oil base, especially vaseline types will weaken the condom greatly. Also as granny said, could be the box!(the condom box) or also if you trim your base and have the little fine hairs poking away, they can be quite sharp and poked their way through. Just a thought.