I think more common than you can believe. low/mid level management is incredibly stressful and hard to build a proper support framework around and hard to do if that framework isn't there.
After you asked, I spent a bit of time thinking of similar situations (high low/mid turnover) and I can think of 2 or 3 other instances I know of personally. Anecdotal I know, but there it is.
I've seen turnover that bad at past employers. In some cases it continued for years after I left. I think it depends on the company, not necessarily averages.
Ditto for a place where I used to work. Unfortunately for me and my colleagues, the reason for the high turnover was that they kept hiring good dev managers who all quickly identified the projects' major problems and set about trying to solve them. This had the unfortunate side effect of revealing the incompetence of the TD, so they had to go quickly.
I know turnover in engineering is usually high (if people are moving around every 18 months to 2 years)... but 600%?