Twist pink image on timeline when extending clip to left

When I extend a clip to the left, the image on the timeline changes to something twisted and pink and then Shotcut crashes. What is that?

It is making a transition. That is the default behavior of overlapping clips on the timeline. If you are trying to add unexposed footage from that clip whose edge you are dragging, you can instead turn on the ripple function on the timeline toolbar, perhaps temporarily.