Note the chutes are clean and ladder in stowed position - So driver isn't washing out. Concrete has very high slump - Might be flowable fill. My guess; they are attempting to reinforce the washed out shoulder of road. Driver failed to put on extension chutes and got too close to the edge. Shoulder gave way, driver will need another job.

Also, mixer drums turn CCW while unloading which would shift weight of load to the passenger side of the truck.