How and why did this Excel problem happen?

I often work from home, so will email files from the office to my home pc so that I can work on them.
This afternoon I was meant to be working in an Excel file, creating custom toolbars that were meant to change as you moved from sheet to sheet. I didn’t get that far before I left for the day, so I quickly created a toolbar with the names of the macros I wanted to run from it, so that I wouldn’t forget, then I zipped the file and emailed it home.

At home, I use Outlook Express. I downloaded my work email, but didn’t read any of the messages, although it looks like I clicked on the relevant one.

I have just opened up Excel for another reason, and the custom toolbar I created at work was waiting for me.

My questions are, how did this happen if I didn’t open the Excel file with the toolbar in, should it happen, and is there anything I can do to stop it?