Covid messed up maintenance planning, which is based on hours and cycles. Some aircraft didn’t fly for a year, now most aircraft are due C-checks at the same time. Prolonged storage also resulted in issues such as corrosion, which extended hangar time during overhauls and further disrupted the maintenance schedule.
Better to do it October and November when there are gaps in the schedule, than mid-Summer or December onwards when entire lines of flying would need to be cancelled or subbed out. Some aircraft were done in May/June.
Airlines like BA were able to absorb the majority of the maintenance bottleneck because the resumption of all of their routes has been staggered, so there’s been natural gaps in the schedule. However I believe even they have at least two 777s in “storage” awaiting hangar slots.