Dissolve audio part - causes?

Can anyone guess what I would have done to an audio event for the fade handles to disappear and the event display change (very slightly) to have horizontal lines near the top and bottom of the part (which blocks the view of the grid underneath)?
I found the solution to return the fade handles by searching the forum (Audio > dissolve audio part) but I don’t understand how I got to the point of needing to ‘dissolve audio part’ in the first place! Any ideas?

Here’s a couple of pictures to show what I mean (pink event):
event clip

event clip2

My guess is you had several Audio Events selected and accidentally used a Key Command to turn them into a Part.