I can speak from a trucking perspective. it was a cascade failure for professional drivers.

when covid first hit, supply chains either had too much or too little almost instantly. Things like raw supplies based on JIT ordering either ground to a halt because demand died and so did the modelling, or because demand was too high for the computer model (think a bell curve.) low-boy trucks shipping things like construction equipment and preform concrete werent affected as governments in most cases stepped in with works projects to keep people employed, but trucks in the supply-side chain of things like toilet paper and frozen pizzas ran into trouble in almost the first few hours of the pandemic.

It helps to keep in mind all these things in JIT are connected. they work like a slinky, and anything that holds up the line will only amplify problems later.

frozen pizza is the best example. for example if things like bell peppers and onions could be delivered, but dough could not, then trucks for dough would idle in the yard until their yard-pay expired and they moved to the next job. yard-pay is the money you earn idling after X minutes in a lot waiting for a hookup, and it can be rather substantial. Anyhow, once dough can be shipped again, peppers cannot because the peppers have spent too much time in transit and now theres loss. pepper trucks then go offline for cleanup and you have too much dough and surprise, the onion trucks just automatically dropped you as a customer because you didnt meet a monthly required minimum.

this doesnt even cover the hell of intermodal port shipping. containers full of perishables rot because cranes are frantically unloading COVID masks, and now those containers are offline for cleaning. containers full of COVID masks cant be unloaded because theres a backlog of Hydrogen Peroxide for a floor tile company in Sheboygan that went out of business eight minutes ago due to shipping constraints, and if its not unloaded and chilled the entire port will explode into flames. in the yard, eighty-one trucks have been on yard-time for nearly 3 hours and a third of the trucking companies paying that yard time will be bankrupt because of it by the end of the day, but they have no choice. Trucks and drivers that went unpaid for weeks are now just taking up lot space until someone figures out how to remove them and the city will likely have to foot the tow.

Later in COVID as cities enacted mass casualty plans they basically absorbed any and all refrigerated 53' trailers they could find as a temporary morgue. now it doesnt matter how many pizzas you wanna sell, the toilet paper company has bought up all your transport market and the city just left you with no refrigerated trailers to use. Once cities are done, you generally scrap those trailers as they cant be used for food again.

Finally, you have trucking firms that went belly-up during the shutdown for any number of reasons, including over-leveraged in JIT style software or a single customer. these truck drivers either left the job to do something else because professional driving is a pretty thankless gig, or retired because the average age of a professional driver is in the 50's. now that we need drivers we have no one to perform necessary training, so it doesnt matter how much cash we throw at highschool kids.

yard conditions also play a role. factories that manufacture bacon might be running at 10% capacity due to covid infections, so trucks show up and idle because the past 3 logistics managers either died or went home sick and nobody in the front office knows what to do. these factories cant or wont shut down because theyre considered supply chain critical (s/bacon/disinfectant/), with "hero" workers and whatnot. shipping companies might also decide to drop you as a customer due to insufficient COVID precautions or an overwhelming amount of COVID infection.