When Is a Runner Considered "Passed?"

[Fan10]

I was sitting in the stands at an NCAA D1 game last week. In the bottom of the 9th, the road team had a one run lead with one out and a runner at first. The batter hit a "no doubter" over the LF wall for an apparent walk off homer.

Sitting in the stands, I was looking at the ball, and as I looked up, I noticed the 3B coach start frantically waving his arms and screaming at the two runners. The next thing I know, the dugout for the defensive team started frantically pointing at the runners and yelling at the umpires.

Since I didn't see what happened, it's not right for me to comment one way or the other. What I was told by some other fans in the stands was that as his teammates were wildly spilling out of the first base dugout in celebration, the batter started going towards the mound before going to home plate and that the 3B coach got his attention and got him back on the base paths.

After both runners came around and touched home, the umpires got together, and the homer stood. So, my question is when exactly is a runner considered to have been passed? If what I was told is correct and the batter goes over towards the mound while the runner runs the bases, is the batter considered to have passed him if he goes "in front of him" but not directly in front of him? For example, if the batter goes toward the mound and is parallel with the shortstop position (near the mound) while the lead runner is rounding 2B, would the batter be considered to have passed him because he is in front of him--but just not on the base path? Or, is the rule that the trailing runner has to go directly past the lead runner to be considered passing him?

I have never thought about this situation before because to me, passing a runner always seemed to be very obvious. I had never thought of a situation where one veers from the base path possibly creating a passed runner situation.

Thanks in advance.