Well as the reporter said - OMG!
Good thing the finish line structure wasn't more substantial. Local hay carters wouldn't know if they're over maximum legal height or not.
If that finish line was as high as the legal height for a hay truck (which from a quick google is 4.6m), then that truck was over the legal limit by a good metre. So personally I suspect it was something else, either (a) the finish wasn't high enough or (b) the truck wasn't meant to be there.
Actually now I think about it, a bigger concern is how close the truck was behind the cyclists! By my watching of the video, it's only about 3-4 seconds behind. What if the cyclists had had a spill? Weren't there road closures?
I was there today, and though I didn't see the crash itself (I was seeking the shade between each pass of the race) I did get a close look afterwards. It looked to be about 3.5 metres clearance underneath to me, which would make the truck legal.
I wonder if the race organisation had the appropriate height restriction signs and detour routes in place? They haven't said a word about the incident yet, which suggests they may have something to be embarrassed about.
No issue there. Those cyclists in the background were the Team Sky riders on a training/reconnaissance ride, using the road while it was open to general traffic while the race was out on the first of two 56km loops which made up the stage. The TDU/WTDU road stages operate under a rolling escort, not a full road closure.
The banner was still being repaired when the race came through at the end of lap 1 for the intermediate sprint, and during the second lap it was reinstalled (to polite applause from the crowd, like when the pitch covers are taken off after a rain delay at the cricket) in time for the stage finish.