When you run an export you are presented with several output options:

User-added image

Choosing Excel will strip the carriage returns from the output.
Therefore, when you open the file in excel, the contents will fill up the cell and move to the next column to the right instead of moving to the next row.

Note that if designing your own export, large text block fields should be put at the end of the list so they can move to the right without impacting the other columns.
Also, another way to fix this issue is to remove the carriage returns in the Excel document.