Why has windows perfomed a ‘recovery of storage folders’ on opening WLM (twice) It messes them up but most mail seems to be there

Opened wlm yesterday6 - message up saying windows was 'recovering storage folders' - all mail for past few days back in 'unread mail' and storage folders mainly there but contents jiggled - took me ages to sort it all out and then this morning I got the smae message - this time only the messages from yesterday came up and the storage folders look OK. Any ideas?