Have you ever tried to open an excel spreadsheet and received a message that another user is in the workbook? Every time an excel spreadsheet is open, it creates an instance behind the scenes with your log in information.
Sometimes Alteryx is a little too fast to write out information to an Excel spreadsheet. If you want to output data to multiple sheets in the same workbook, I recommend using the Block Until Done Tool. This tool "holds" your data until the first portion of your workflow is done processing. Then it writes out data to the first output anchor, then when that is done, it writes out to the second and so on.
People ask me what if you have more than three outputs? Use more Block Until Done Tools!