My all bret saison does the same thing. I just assumed it was from the slight injection of oxygen and agitation due to the action of adding the hops.
Primary: Old Bootlegger Imperial Porter, Mill Worker's Breakfast Oatmeal Coffee Porter
Secondary: Oud Bruin, Berlinner Weisse, Apfelwein
Barrel: Whiskey :P
Kegged: Mill Worker's Breakfast Base
Planned: Pure-T Pale with Citra, G-Town Brown 2.1, Muddy Banks Irish Red