Neck crowding is when the neck of a case is expanded past its tolerance range for a chamber, by the use of oversized bullets, for that cartridge. Ie, if you use a .312 or .314 bullet, in a .30/06 neck, ( which was made for a .308 proijectile,) your .30/06 chamber neck, will have less "tolerance" with the expanded new neck diameter of the loaded case. Hence, the neck, on firing, cannot expand as much as it could with a .308, and so the neck gets "crowded" by the normal .30/06 chamber neck diameter. This loss of tolerance, can lead to sharp increases in pressure. ( we are talking of only a few Thou of an inch, here.)
The larger diameter bore/grooves, with the Lower diameter bullet, will allow both "windage"( improper seal of gas by the projectile) and also inaccuracy, because of lower pressures behind the bullet. Flat based bullets perform better than boat tails, because the flat base will "bump up" to seal the slightly larger bore/grooves.
To correct this problem, one can either "hone out" the neck to take a proper .314 plus neck thickness cartridge, allowing the correct tolerance between Cartridge and Chamber, or reduce the powder charge ( and thus the pressure.).
Hope this clears the air on the problem.