As others have said, first that isn't that big a turn over in 3 years for a school of that size, considering there's a new head. Generally 1 or 2 teachers will move on each year anyway, for a variety of reasons, so it's only slightly higher than average I'd say for a 3 year period.
Whilst I understand this can seem alarming to parents, it isn't always simply a case of the new Headteacher making it an unbearable place to work so now everyone wants to leave (although sadly this could be a possibility). Most likely reason is that those staff who were there for years weren't happy with the changes (that doesn't necessarily mean the changes are bad though, they could have been essential and those staff too comfortable in the old system). Once one person leaves, this can cause their close colleagues to consider it too as the place doesn't quite feel the same anymore (especially if they were also close to the old headteacher too).
It could be that they're experienced (and therefore expensive) teachers who have sadly been edged out as budget cuts are so bad the school simply can't afford all their wages anymore. This isn't always the fault of the new head, often their hands are tied. An old head who is close to those teachers and therefore has loyalty towards them will ensure it doesn't happen, but when there's suddenly a new head with no prior relationship with those teachers it's very easy to suddenly give them the most difficult classes, extra responsibilities and create an atmosphere where those staff start to consider if a change would be good for them. Then they can be replaced by cheaper ECTs (newly qualified).
It's so common when there's a new head. Once they have bought in their own people and stamped their mark on the place things generally settle down but a 2-3 year period of change is pretty standard with new management.
Unless you personally have concerns about how the new Headteacher runs the school and conducts themselves, I wouldn't read too much into many of the 'old guard' leaving. Whilst it's likely to be because of the changes, it's not necessarily because of the Headteacher themselves being bad if that makes sense.