Research promotions in the middle of post-doc contract
When I recruit postdoc it’s under the assumption that they are taking on a transitional role. In some sense I believe it’s in both our interests to have them leave sooner if possible. It’s good for them (pay bump, tenure career starts etc), and for me it helps since it signals to future postdocs that I’m able to get them on the right track to a faculty position. 

In some cases it’s the opposite: a person has secured a faculty position straight out of PhD, and then postponed their start date to take up a one year postdoc (in MSR/Facebook research etc, or with another research group). This is usually done to build up one’s professional network before committing to a department.