Seems like only two things could cause something to change all of a sudden...

1) Conditional Formatting

• Select the entire sheet and Data >> Data Tools >> Data Validation
  
• Is anything there? If there is, it would mostly likely be based off of some type of trigger.

2) VBA

• Same with VBA. Something has to act as a trigger to fire the code (if there is any)
  
• Select Alt + F11 and look through all of the modules to see if any code exist in your workbook.

http://www.rondebruin.nl/win/code.htm