Why do some bacteria have an asymmetric replication?
The replication fork itself is always asymmetric. This is true for all organisms and is due to the fact that DNA polymerases can only add nucleic acids to the 3'-OH end of DNA. This leads to different mechanisms for the replication of the so called leading and lagging strands. You can find more details on wikipedia https://en.wikipedia.org/wiki/DNA_replication

Most Bacteria have circular chromosomes that they usually replicate using two replication forks. Both forks start from the origin of replication and progress in opposite directions to meet at the replication terminus which is opposite to the replication origin on the circular chromosome. Each replication fork replicates about half of the chromosome. We talk about the left replicore and the right replicore. I assume that your teacher was referring not asymmetry in the replication fork itself, but to asymmetry in the size of the replicores. In B. subtilis the left replicore is 196 kb longer than the right replicore (J Bacteriol. 1993 Feb; 175(3): 741–749.). I do not know whether there is any evolutionary explanation for this, but mechanistically it is caused by the fact that the terminus of replication is not placed opposite to the origin of replication on the circular chromosome.