When I add a comment on the main site, something breaks such that IE8 drops into compatibility mode for the site. This has unfortunate side-effects like removing the bar across the top containing login name, rep info, badges, links, etc, but the root cause is always adding a comment. That is the only action I've found that causes IE8 to refresh the page in compatibility mode.
This is a known bug. Apparently there is some difficulty reproducing the problem (it doesn't happen for everyone). If you can help the developers understand why it works for them and not for you, that would help to fix the bug.