two things come to mind.
the beerline warmed up inbetween pours, therefore the first glass would be foamy until the beerline cooled down to beer temperature.
the sankey was overcarbed, or you didn't cool it down enough. the head pressure would build up in between pours, so the first pour would come out very quickly, causing the foam, but the headpressure would go down, so the successive pours would be ok, but inbetween pours, the headpressure would again increase as more co2 would come out of solution.
solution: make sure your keg and beerline are at serving temperature, and that the keg is at proper carbonation level.
at least that is from my perspective, which could always be wrong. haha. i hope this helps though.