Excel spreadsheet undoes/reverts to previously saved state

A very unusual problem haunts my colleague
From time to time when she works with Excel 2010 (numerous workbooks, some of them quite large), the computer "decides" to undo everything that was done since the last save. You can see on the screen how past actions are being undone one after another. After that they cannot be redone back, as the button is grayed out. I've never seen such problem on any other PC I've worked on, but I witnessed with my own eyes how this happened on hers.

A Quick look at opened workbooks didn't show any VBA macros. I didn't look inside every worksheet though--as there were probably a hundred or so of them. I also suspect this has nothing to do with macros as running a macro immediately clears Undo list. This happens not too often, probably once in week or so and I cannot replicate this because it happens sporadically and just on one PC.

Just for clarification - this happens when file is opened. Not when it is saved and reopened (which was a problem some time ago with shared workbooks).

Any ideas what causes this?
Where should I look for clues?

You may want to do your own versioning - Saving a new file as FileName_Date.xlsx

I'd also check the autorecovery settings to see how often it's saving / where and consider turning it off.