3 things come to mind:
1. Packing too hard.
If the tobacco at the bottom of the bowl is too dense the moisture which results from combustion will collect there.... remember all the smoke goes THROUGH the tobacco at the bottom to get to the other end. Ever seen a smoked cigarrette filter or cigar butt? Same principle... best to give it plenty of airspace.
2. Smoking too fast/hot.
Just about any tobacco will lose a lot of what it has to offer if you let it get too hot. It can get pretty wet and nasty if you "hotbox" them. (That might be better left for more intimate situations). I think slower and cooler is the ticket... You'll appreciate this more if you ever start getting into flake tobaccos. They just don't work well if you puff them too hard.
3. Too moist
If the tobacco is too wet, the resulting condensation will build up in the heel and all the "gurgliness" equals sourness before too long.
In general, err on the side of too LOOSE when packing. You can tamp it down if its too loose or want a more intense concentration to the flavor. Harder to unpack it once it's lit. I just gravity fill the first part of my bowl- almost no pressure on it at all. Then as I fill it up I'll pack each layer a little firmer. It just takes a little practice is all.
The "cooler" smoking is good thing to cultivate especially IMHO. In addition to minimizing tongue-bite, the complexities of the different tobacco blends are more accessible. Kind of like food. If you eat it REALLY hot, it's still good, but you can miss some of the flavor and subtleties. It's just to HOT. When it cools down a little, however, you can really get a feeling for the flavor. This too takes a little practice.
One afterthought. If your pipe is really cruddy inside, this can be a detriment as well, no matter how good your technique is.