Anyone one else wipe clean credit-card magnetic strips

Just by having them in her possession, my wife manages to wipe all information from the magnetic strip on her credit cards, ATM cards etc, the latest lasting less than two weeks before it was rendered unusable. The bank seems quite surprised by this, so I guess its not a common affliction but does anyone on this board have a similar effect on their plastic?
Can anyone offer a possible explanation as to why it happens?

Look for a large magnet or a source of a magnetic field in her purse. Read this… from Cecil.

Thanks Nivlac, most interesting. I’ve checked her purse and, while I have no idea what eelskin feels like, it does have a magnetic clasp so I guess that’s it.