Using priming sugar to carbonate a keg makes it just a really big bottle. It needs to be kept at fermentation temperature long enough for the yeast to eat the priming sugar, and then chilled long enough for the CO2 that was produced to be absorbed.
Otherwise...If you didn't cold-crash or use finings - or just let the beer sit in the fementer undisturbed long enough - before kegging, there will be yeast and fine matter that settle at the bottom of the keg. And that's where the end of the dip tube is, so you will pull some yeast-rich pints until it clears out...
Cheers!