While some might say that the most compelling evidence that a project manager is not doing well or has lost control of his or her project is when the team starts leaving, it is not true. There are many reasons a team leaves regardless of the job the project manager is doing. The best of project managers can only overcome low wages or the attraction of a competitor's higher benefits just so long before the team succumbs and leaves. The project manager may fight with upper management to get higher wages or better conditions to no avail. When upper management believes that project staff is interchangeable their response is "let them go, we can hire more", and the project manager is left with constant churn and deadlines missed. And it may also be the natural result of a recovering economy providing new job opportunities for the pent-up demand of employees who have been dying to leave for years, not just as the result of the current project.
Here's a positive spin on team members leaving. According to Tuckman, the fourth level of team composition is "Performing". Most teams, once formed and composed operate in the "Norming" mode and that is where the project managers wants them to be. Some teams (and I've been on them) move into the "Performing" mode wherein the members of the team pretty much act as one, sometimes to the exclusion of everything else but the team and the project. This is great for productivity and creativity, and the project manager need do nothing but watch since the team pretty much manages itself. The downside is that when, for whatever reason, a member must leave, the team disintegrates. They cannot achieve the same "high" they had while in the Performing mode, and each member typically leaves the team and usually the company in short order. Nothing can be done about it. It's just human nature. And the project manager previously watching the team perform wonders, now wonders what just happened. And it always happens. Sooner or later someone will leave - retirement, following a spouse to a better job, required transfer within the company, etc. The project manager cannot stop the team from sliding into Performance and cannot stop the eventual end, and can only hope the project end precedes the team end.
Yes, blame can generally be placed at the project manager's feet when the team decides to leave en masse over a short period of time without any outside instigation (such as a ten percent reduction in pay across the boards, or a requirement in IT for everyone to put in an additional ten hours overtime without compensation). The fault is usually in not listening to the team. Nothing will chase a person away faster than being not listened to which can be interpreted as lack of respect, a signal that the team member is not important, a sign of project manager arrogance, an indication that the project manager and upper management feels that the workers are just a cut above automatons and can't think for themselves, or all of the above. This condition occurs more frequently when economic conditions are in a down cycle and jobs are scarce so people put up with more just to keep their jobs allowing management to be more arrogant and demanding. When project managers mirror this attitude, they get what they deserve from their people.
Incidentally the definitive indication that a project manager has lost his or her project is when someone outside the project tells the project manager about something going on inside his or her project that he or she didn't know about.