Exactly what the problem is, nobody really knows - the corruption seems to occur when equations and textboxes get tangled up in a way that shouldn't be possible. In certain circumstances, certain sequences of actions corrupt what will be saved as the document,
though not the in-memory open document you are working on, so that you only become aware of it when you next try to open the document.
Without any knowledge of what triggers the corruption, for which there is no evidence on the working document surface, it is almost impossible to know how to stop it happening again. However, if you are afeared of it, you could try saving, taking a copy
of the saved document, and checking if it will open, and only then closing the original if you know the later version is good - what you do if you find that the saved version isn't good is harder to suggest.