Block data error, reindexing blocks Bitcoin QT finished syncing this weekend after d/l for Windows 7 on Thursday.

Today, probably 2nd or 3rd time running it, an error came up right away that index could not be found.

What would cause this to happen, and why would it have to reindex all the blocks on disk?

Thanks, Mike
When I've had this happen it's been because of a non-clean shutdown. Something about the disk state ends up being inconsistent and this (understandably) results in the system wanting to verify all the information it has on disk.