It's a combination of the pingdemic, driver CPC (Certificate of Professional Competence) training and Brexit (probably in that order).
HGV drivers must have completed 35 hours of training and renew their licence every five years (every year for those over 65). The renewal has to be done by paper and mail, it cannot be done online. Some drivers have had difficulty completing their training and although an extension was provided for those who could not complete their training by August 31st 2020, that facility was removed. This has caused many to be unable to continue working.