That's not how variants come about. It's completely normal and natural for viruses to mutate, and they do so all the time. When a mutation proves to be able to cling to human cells more effectively, then that new genomic sequence becomes replicated and spreads faster. The lamda variant, which is the newest variant, appears to have a genomic sequence that is vaccine resistant. In other words, it has mutated into a sequence that can continue to spread irrespective of the proportion of vaccinated people.