Sometimes event groups will open a "add new action" window when clicking on them. When it happened to me the groups were closed, and I could not find a way to open them. This might also correlate with groups being at the top of the sheet.
Someone else asked me if this ever happened to me, so I thought it must be common enough to report, even if I can't figure out how to reproduce it.
Ok, it just happened with my bottom group, so it's not related to position. It also happened to an open group, so they don't have to be closed. Interestingly, the entire bar becomes a link, even though with most groups it's usually only the text that responds.
At a guess I'd say an event's 'add action' link is being rendered or set to the same position as the group link, David.