Maybe they thought that they were pretty well clear of the fight and were forming to march out. Or their officers were total idiots and were thinking that the old Napoleonic charge in column would carry the day (night). That other section gun, at 100 yards, should have had great enfilade fire into the flanks of the center and tail of that federal column. I'd also take that "20 yards" with a grain of salt, but even if you stretch it to 50 yards the spread wouldn't be all that great. At 20 yards it would almost be like a fragmentation grenade going through them. No idea how they kept going, other than maybe it was less dangerous to go on than to try to go back.
I'm with the Norfolk Light Artillery Blues, NCWA, in northern CA. We can field a 3" Ordnance Rifle, a steel 12 pound Napoleon, both from Steens. And a 10 pound Parrott, don't know who made it.
Here are two videos of that night fire: http://www.youtube.com/watch?v=t6Mlas11xWA
http://www.youtube.com/watch?v=ihRzuXlpOyQ
There was a bit of confusion between us, the federals, and the fireworks people. But it worked out. We started with mortars about 50 yards behind us, then our gunline fired.