It's a bit like being a golfer cursed with a case of the Yips (an inability to sink even simple putts).
I'm sure it happens to us all, and I know for a fact it definitely happens to me. I get a series of poor landings (like yourself, either uncomfortably firm or off-centerline - which I find particularly disconcerting) maybe every 5 or 6 weeks, and it lasts for anything from just a couple of days, to maybe a week or more. Fortunately all of them are inside the touchdown zone (quite happy to let pride take a fall in order to avoid a runoff), but not pretty to watch.
I find that they occur far more often when I'm feeling tired (not fatigued in the true sense), as I seem to have a little more difficulty concentrating. Maybe it is down to over-compensating for my tiredness and trying to concentrate too hard because I'm aware of being tired, and instead of flying nice and relaxed, I end up tense. Either way, the first poor landing makes me more conscious of trying to redeem myself with the next one, and yet again I end up tense instead of relaxed and thus the cycle begins.
Silly I know, but that's how I would describe my own experiences.