As others have said, the "mass exodus" happens all the time. Yes, it can be temporarily disruptive, but (generally) it should not raise concern. During a mass exodus, you and your child might be wondering: "Are we in the right program? Should we move gyms?" In my opinion, these are questions you should always be asking yourself!
Sometimes gymnasts outgrow their gyms..Sometimes gymnasts dislike their coaches...Sometimes gymnasts just want to do gymnastics with their friends (who happen to go to other gyms)...Sometimes parents prefer a different training schedule...Sometimes parents have tuition constraints...Sometimes...Sometimes...
In summary, no gym is perfect, and it's important that you and your child continually assess whether your gym is a good fit. There is nothing wrong with staying or leaving as long as the decision serves your family's goals.
Lastly—as I'm sure you've experienced—mass exoduses happens all the time in other parts of life. For example, there is always a group of people boycotting businesses, quitting jobs, leaving the country, etc. If your child is distressed about the mass exodus happening at the gym, use this as an opportunity to teach her about emotional resiliance during times of disruption. This lesson will come in handy later in life!
P.S. The only time when you should be concerned about a "mass exodus" is if people are leaving the gym due to dishonest business practices or abusive coaching.