Yesterday in an NAIA college game, there was a runner on 1st. Batter squares early to bunt, and third baseman and second baseman(no this is not a mistake), come charging hard and end up about 10 feet in front of the pitcher's mound before the pitcher delivers. The manager for the offense said they cannot do that. I looked dangerous to me, but I cannot find a rule that prevents the defense from doing that. Is there such a rule? And what is the penalty?

Charging on a bunt situation is a universal defense tactic, used at all levels. Any coach who says it's illegal is either a blatant neophyte or just yanking your chain.

According to the BDR, the NAIA uses OBR with a few modifications. None of the modifications listed prohibit the action you noted. OBR does not prohibit the ction you noted. OBR 4.03(c) "Except for the pitcher and catcher, any fielder may station himself anywhere in fair territory"

The only way you can have any type of interference is if during the pich the fielder is distracting the batter in the umpires judgement. This would include yelling, or waving his arms. This probably would not happen at the college level. Because yo definitely would have the opposing pitcher putting down the first batter.

There is a way to deal with "Bush Baseball". And a good umpire understands this and uses the slap on the hand technique.

Call time if the fielder is using these actions and warn them for unsprtsmanlike behavior. There would be no pitch. No award.

If they are charging to make the play it is good baseball. You now have 1st and 3rd covered. Without taking the 1st baseman away from keeping the runner on. Do not take this away.