Audio Events Grouping Themselves?

I have a question that’s been bugging me for quite awhile now!

In Cubase 6.5, sometimes when I select an audio event,
one or more other events get highlighted at the same time (and behave just like grouped events).
I check to see if they are grouped, but they’re not, so ungrouping them (ctrl + u) does not help.
This happens a lot randomly in the middle of an editing process.
I must have done something accidentally to cause this but I can’t figure it out!

This seems like such a basic question, but I just coudln’t find solution.
Please help - thanks so much!
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