Viruses mutate continuously and this can lead to new variants. Of the thousands of variants that are emerging on the SARS-CoV-2 genomes, it is reasonable to expect that some variants can eventually achieve biological advantages and be more transmissible, clinically aggressive or resistant to treatments or vaccines. Randomness and social behavior can also explain the predominance of a particular strain.
Update (March 2023):
For more information on SARS-CoV-2 variants see: