So weird. That used to happen to me all the time when I first took up a pipe and now it never (or rarely) does. It can only be one of a few things:
1.
tobacco cut
2. fill technique
3. pipe geometry
New pipe smokers often gravitate to shag cut burley/virginia blended tobacco commonly seen in aromatics. It does include lots of little flakes, chips and crumbs that can easily find their way down the stem to your mouth.
If you smoke a lot of shag consider developing a Frank (German) fill or Airpocket fill. Shag and ribbons work really well with these fills and keeps tobacco off the airhole. That, or try a little squished wad in the bottom of the pipe before commencing with a conventional 1-2-3 fill. Should you graduate to flake tobacco that problem will probably vanish (but it'll be replaced by other little problems that come with flake tobacco.
) - I mostly smoke flakes and I hardly ever get tobacco shot down my throat.
If your pipe has an airhole that looks like the entrance to the Holland Tunnel that might be a problem. A lot of cobs have gaping airways and that could cause tobacco gulping according to the tobak you smoke and how you fill.
So, this is all very interesting. Whatever you do the problem will go away with time and experience I am sure. In the meantime, don't spit the bits when anyone is watching.