Job of a product manager is strategic in nature where quality is of paramount importance and at the same time it (quality) is difficult judge on shorter duration. Product Managers run marathon and not sprint, it is important for him/ her to be focused for longer duration and deliver quality output consistently, a challenge I guess many struggle to deal with. That's where you see these Jumping Jack product managers who habitually stays for less than 18 to 24 months on a job (2 or more jobs) or who have never experienced a complete life cycle of the product.
Is it that difficult to stay for 24 months, or experience a complete product life cycle. Wait and see what happened to the decisions that you made last year or in initial stages of product development. What happened to those critical decisions you made in growth phase of your product and how long did your product stayed in maturity stage. Product Managers who do not stay long, would never ever be able to experience these situations.