What causes pillarboxing to occur part way through a movie?

On very few occasions, my canon optura generated video when being imported to iMovie will have a series of clips, pillarbox. The overwhelming majority of all my movies and their clips never pillarbox. I have the camera defaulting to 16:9 and iMovie import preferences set to widescreen.
I might on occasion have been trying different camera modes/actions during a vacation, etc. and somehow get out of 16:9 or ?????????
Main question is what causes pillarboxing in the first place and from there perhaps I can figure out what I have done to trigger it.

Uncheck "iMovie HD/Preferences.../Import/Automatic DV Pillarboxing & Letterboxing".
Check these out from Matti Haveri: