If you have Replay or recording turned on, the scanner is saving audio data to the card every time it breaks squelch. Cards have a finite number of write cycles before they wear out, and when thousands of files are written to the card every day, it can wear them out quickly. More expensive cards are rated for more write cycles before failure.
It's also possible that you are corrupting the card by not shutting the scanner down before removing batteries. Killing power in the middle of a write can also corrupt the card, and should be avoided.