Its just the evolution vs revolution periods... In any field 99% of the time advancements are slow and incremental, than a new idea sparks a revolution and you get a lot of advancements in a short period of time. This is just the nature of scientific progress.

In the early 20 century new ideas in physics and mathematics sparked a revolution and we are slowly unraveling the benefits of it ever since. For 40 years we had evolution in the AI research field, now we have a revolution, I do not know when it will die down but after a while we will just start to build over the ideas we discovered and a long period will pass before we do another jump. Same with psychology, economics, biology...