Good question. What's happening is that drivers are also limited to a 14-hour on-duty period (this would include the maximum 11 hours of driving) before a mandatory 10-hour rest period. Because of traffic congestion, delays in picking up loads, lines at terminal gates, and long times to unload the truck at understaffed warehouses, drivers are cutting their days short because they don't want to risk running out of the 14 "duty hours" while they are stopped somewhere and can't lay over for the 10-hour rest period.
Think of a driver who drives for 10 hours and then backs up to a loading dock. When warehouses were fully staffed and operating efficiently, they could unload the truck in 2 hours. That gave the driver 2 hours to find a place to lay over for 10 hours of rest.
Today, that warehouse is understaffed and takes FOUR hours to unload the truck, not two. So the driver reaches the end of his 14-hour "on duty" window while he is still parked at the loading dock. He's in violation of his hours-of-service rules if the property owner forces him to leave.
So in this case, the driver cuts his prior day short and makes the delivery the following day when he knows he has plenty of time on the clock to finish the delivery and get moving again.
And AC is only scratching the surface of the rules and restrictions that truckers have to deal with. To say nothing of port authority rules and rail traffic restrictions. Plus the cost of fuel oil for ships... or the shortages of shipping containers. The list goes on and on. Our supply chain is in shambles.