S-shaped learning curves
As per A Umar Mukthar's comment, the phenomenon is known as an S-shaped learning curve. They have been a known phenomenon in psychology for many decades, and were originally attributed to trial-and-error learning sets (Harlow, 1949). Harlow defined a learning set in the following manner:
The monkeys learn how to learn individual problems with a minimum of errors. It is this learning how to learn a kind of problem that we designate by the term learning set.
Meta-cognition and intra-individual performance variability
Harlow's explanation has fundamentally not been changed too much, except perhaps that in modern cognitive research, we now call these meta-cognitive strategies rather than learning sets.
When first encountering a novel problem, participants will adopt one strategy and work to improve their performance on that, but eventually they will discover other strategies. When participants shift strategy thus to an unpracticed strategy, their performance declines, then improves again.
Because all strategies for the same problem tend to share at least some overlap, as participants explore an increasing proportion of the space of possible meta-cognitive strategies, shifting to a novel strategy requires decreasing amounts of new practice. The magnitude of the performance decline associated with strategy shifts is therefore gradually eliminated with prolonged practice (Murre, 2014).
- Harlow, H. F. (1949). The formation of learning sets. Psychological review, 56(1), 51.
- Murre, J. M. (2014). S-shaped learning curves. Psychonomic bulletin & review, 21(2), 344-356.