Mostly misuse. The studies are all nonsense AFAIK, they rely on the participants to use the condoms correctly, which predictably doesn't happen. Problems the studies mention, like slippage and breakage are the result of misuse, probably due to choosing the wrong condom size.
The only reason for a properly used condom to not work would be a manufacturing defect, which should be extremely rare, certainly not 2%, that's plainly ridiculous and immediately disqualifies any study that claims so.
