In summary, it took the form of a simple game, for which many players deduced the optimal strategy. They then modified its behavior, without notifying the subjects, so that the optimal strategy remained the same, but was less successful, and many players strayed from the optimal behavior. So were they exploring? Had they simply lost confidence in their earlier assumptions about how the game worked? We don't know why they strayed, because "while the answer to that is beyond the scope of this study, Krajbich said, it likely takes a lot of mental energy and planning to always make decisions based on your knowledge of the environment."
Notice, in that quote, the willingness to speculate about a cause despite an experimental design that could not deliver evidence on the issue... (To be fair, I probably would not have noticed that inconsistency if Krajbich had picked a putative explanation that I speculate is more probable.)