Turnover...poor pay...bad working conditions...less than enlightened management at the local hospitals...and within some of the operators.
The stress of flying at night in marginal weather conditions with less than adequate weather reporting and lots of wires and rocks to run into....adds to the problem. Compound that with some of the new cars today costing almost the same as your annual income....and trying to raise a family on yer wages....well guess there might be stress there. Add in the fact that all it takes is a squawk by a Medical Crew and you are out of that job like a cork from a shook bottle of hot champagne and you can see why there are lots of job ads.
It takes a special kind of pilot to fly EMS....what should be a really enjoyable job can oft times be anything but that.