You can simply ask the school about the reasons why the teachers left. Chances are that they were fairly young women. I find that any group of young female workers tends to be fluid. They are moving cities looking for new opportunities, going back to school for different training, getting pregnant and taking maternity leaves....... I used to work with almost all young women and our manager was constantly trying to fill positions for all of those reasons. She used to mock-threaten people from sitting in the same chairs as colleagues who had recently become pregnant, as if it were contagious.
You can also ask some of the other parents if they know of any other reasons for the turnover and listen for indications of more serious, disturbing problems like an unhappy work environment (low pay, employee abuse etc.) or a problem employee.
I would also ask to spend some time observing the class or even volunteer at the school, if that's possible. There's often a lot of information (okay, gossip) that you can pick up if you are on any of the parent committees, if you have time for that sort of thing.