I've come to the conclusion it is a combination of staleness and tiredness.
I have a two mile commute uphill on a hardtail mountain bike. It has felt a bit of a drag for ages, probably ever since my summer holidays. Last week I was working away for 3 days and had longer hours on the other 2, which meant I got very little opportunity to exercise. I didn't mind because I was supposed to have a time trial on Sunday. Saturday was an easy 30 miles to check the TT course, Sunday TT was cancelled for safety reasons due to rain and standing water, Monday I was working away from home again. Early Tuesday morning I did a hard session (2x20s) on a gym bike, went home and did my first sub-9 minute commute since July - 1 second off my best ever.
I reckon it was a combination of more rest last week and not having done the commute for about 8 days. Your brain spends a lot of time trying to work out novel ways of stopping you pushing hard - too tired, can't be bothered, sore legs etc are often self-protection mechanisms that we may need to get past to improve. A reminder if one was needed about following the oft-given advice to mix up your training to keep it fresh.