
Those two, specifically- the highlighted one, and the other one occupying the exact same space. They're vibrating, trying to path off to an excavator, but... failing. I initially thought it was because I had a single-square-wide pathable space there- but then I widened it (you can see one of the excavators finishing that up), recovered the stuck trucks... and more promptly got stuck. It's at the top of a ramp.
Update 4.2 | v0.8.6c (b612).
Reproduction unknown, but save attached and it's verified to persist through reloading. The other trucks might be in slightly different positions when you load the save, I did run it for a few seconds- to verify persistence after load- before taking the screenshot.