Reasons for the sudden uptick in COVID cases .....

...besides the fact that COVID-19 never went away in the first place.  It's been still lingering around here and there.

I think the biggest factor is simply population density, the fact that one has to converge onto a lot of the same areas as everyone else:  shopping at the same stores, riding the same buses and commuter trains along designated routes, dining at a lot of the same restaurants, just all-around being given only so many choices of where to go to conduct certain types of business.

For example, getting a new drivers license or state I.D..  There are only so many branches of the motor vehicle department, hence whichever one you go to you'll still find yourself standing in a long line behind others.

Personally, I think even more so than whether or not one wears a face mask or has been fully vaccinated this "default" interpersonal congestive bottlenecking is probably what's most responsible for increasing exposure to this (or any other) virus.

That said, I still contend shutting down again is not going to help any.  Because, sooner or later, we'll need to reopen once more (you know, to keep our social and financial ecosystem from totally collapsing).  Then all this will continue happening all over again until this COVID-19 virus finally wanes a bit.

Ultimately, COVID-19 will just have to run its full gamut, no matter what we try to do about it.