Most failures occur due to worn or broken tape components, but tapes can also lose oxide over time; poor storage conditions can exacerbate this oxide loss.
Additionally, tapes can fail due to impact damage, dust contamination or tape drive malfunctions. Logical read/write issues can also prevent normal file access.
To limit your chances of a tape failure, you should store your media in a clean, properly controlled environment. Check your tape backups occasionally and contact a professional data recovery provider as soon as you see signs of a failure.