Maybe this is described somewhere in the documentation, but I haven't found it:
What does it signify when the background of an audio or event is pink??
If it's associated with a video track, it means it was ungrouped (or you have Ignore Event Grouping enabled in Options) and the audio track is now out of sync with the video. You can right click on the audio event and choose Synchronize > By Moving.
But you could have asked this in your other topic as it's related, I think....