From that description alone (which is all we have), it's impossible to even guess as to "why". Rather, what is possible is to suggest that you use all the various (and FREE) methods to protect your data from this or other destructive events. You can use LINK to backup. You can configure for OTA synchronization with a server, and then perhaps also with Desktop Outlook, which you can then conduct PC-based backups to protect your data.
There are many methods to protect your data and at least ease the pain of recovery when this happens. But, as to "why" it's happening, from this information alone, it's just impossible to even guess.
Occam's Razor nearly always applies when troubleshooting technology issues!
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