Infection rates in the U.K. went steeply up and are now going steeply down. There was a similar but somewhat more gradual pattern earlier in India, where Delta first appeared, and I believe in some other countries. That suggests the possibility that a small minority of the population is very vulnerable to Delta. When it first appears it runs rapidly through that minority, then starts to fall when enough of them have either died of it or recovered and become mostly immune.
Further anecdotal evidence comes from a friend whose father and daughter both recently had breakthrough cases, got Covid in spite of being fully vaccinated. That suggests that vulnerability may be genetic. The obvious test would be to see if infections cluster by families more this time than in previous rounds of Covid infection.