Let's do a little mind experiment. You are a virus (any kind, cold, flu, whatever). You need to find a host to replicate in. Your present host has a cough as a result of your presence, and sprays a mist of your virus particles over his friends. If they are close by, a lot of virus particles will find new hosts easily, and your virus family will thrive.
Now what happens when the human hosts start to distance from each other, as a result of lockdown, masks and antisocial distancing?
You have a harder time getting your particles across the gap between your original host and new ones, so there are "empty" hosts all over the place, and the infection/death rate drops (sound like summer 2020?). Then, you (the virus), being stuck in a "full" host, start to mutate. Nothing odd here, viruses do it all the time, thats why there is s different flu vaccine each year. But, and here is the big 'but', the mutants you have produced don't go anywhere, because they can't get across the distancing gap, just like the "common or garden" variants you have been producing....Except one, who has mutated to be more virulent and able to infect hosts easier, with less host exposure, less virus particle exchange and more transmissability.
WeyHey! look at all those "empty" hosts to infect, and your offspring can do it faster and wider even with the lockdown/distance/masks in the way. So all of a sudden, you have a "new" virus variant, that spreads easier and rips through the host population, who have not been infected by the less virulent original virus and haven't been exposed to any form of the virus, so having no marginal resistance built from previous unsuccessful attempts to colonise them.
So pick holes in the story....we are just locking down tighter because of a more virulent virus strain, that arose after a longf period of lockdown, or have I missed something