This sometimes happens when the HTML in description includes characters such as line breaks and comma's. It can cause Excel to have a hard time parsing the file.

If you are trying to edit the product information, use the custom export to remove the description fields (description, longdescription, ebaydescription, etc).

If you do need to bulk edit the descriptions, then just do an export with only guid and description.

If you are trying to edit all of the information, it will be best to update most of the information with one spreadsheet and descriptions with another.