In IE 8, this issue is occurred very often.
In IE 9, it's not always but still often.
Other browsers won't get this issue.

reproduce steps:

1. open any file
2. click A1
3. directly right click on other cell
   • wait for a while, the context menu shows up
   • just wait for another second, the context menu will be closed automatically
4. if not, repeat previous steps again