It makes perfect evolutionary sense. Loads of variations appear in every infected individual. The most effective version takes over and is most likely to be passed on to others.

Most vaccines are so effective that the virus never gains a foothold. With the mRNA vaccines, people still get sick. You get tons of mutations and the original strain is no longer the most efficient because of the vaccine. so now there's tons of room for variants to form which work around the vaccine. Once an effective bypass happens, it takes off and you instantly get a new strain, but more resistant.

At the current rate, we're only a few short jumps away from a completely ineffective vaccine.