There's an easy rule of thumb ... is there an upward trajectory in the responsibility or stature of the roles, or are the moves mostly lateral?

A series of quick lateral moves - this obviously excludes freelancers and consultants who specialize in short term contracts - can be a sign of someone who stays just long enough to either show they're not great at the role and took off before they were let go, or otherwise has something else going on with them that you'd want to look into more. It could be perfectly understandable. It can often not be.