So while we know from witnesses that this did happen, anything about why is going to involve some conjecture. But from what I recall, there are a few reasons.

So while we know from witnesses that this did happen, anything about why is going to involve some conjecture. But from what I recall, there are a few reasons. The English army was arranged in a single line, with large groups of archers alternating with small groups of men at arms. For the men at arms (on both sides), combat was primarily about winning personal glory, secondarily about scoring some loot. The only way to get either is by attacking the enemy men at arms. No personal upside in attacking yeoman archers (other than, uh, maybe winning the battle. Whoops.) So you naturally funnel towards the enemy you want to fight. Remember these guys aren’t marching in formation like Romans or early modern pikemen. It’s a loosely organized mass of individuals. On top of that, the survivors of the cavalry attack on the flanks retreat through the infantry, throwing them into disorder. This also helps condense the infantry as they try to make way for the cavalry. On top of that, getting shot at by clouds of arrows sucks even if realistically you’re not in a lot of danger. Getting out from in front of the archers would feel safer. Plus, even if you’re a smart knight who knows you should be attacking the archers, that doesn’t do you any good unless everyone else comes too. Getting caught our on your own is how you get ganged up on, knocked down into the mud, and stabbed through the visor by some dirt farmer with a dagger. Which is exactly what happened to the guys on the edges of the columns. So you stick with the crowd. Once you’re in the crowd, it becomes packed too tightly for you to change your mind. At the front of the columns, witnesses report the English stepping back at the last moment, causing the leading french to wrong foot their attack and stumble. So it’s chaotic, they haven’t smashed through the English line, but the guys further back have no way of knowing that and keep pressing forwards. As an aside, I didn’t mean to make it sound like the spears were a particularly English thing. Most on both sides would’ve had some kind of polearm. In mass combat a sword is often a backup weapon.