That was Flight of the Falcon, I believe. (Pretty sure)
I did re-read that passage (and I'm planning to re-read the book; I'd forgotten it was Victor Milan who wrote it) and it does explicitly describe his decision to close as a mistake, noting that staying back (not falling back, just not charging head long) would have brought sure victory. We don't see from his view point in that moment, and Tara Campbell attributes it to a need to lead from the front as the remainder of his forces charge.
However, in the opening scene of the book, we she Alex chastise an elemental he is fighting for failing to see the value of tactical retreat, of keeping distance open. Reckless charges aren't in his nature. So why race in? I will offer two rational reasons, and an extra half a human motivation.
In his previous battle, he had faced a massive artillery barrage, which the narrative notes that he skillfully avoided by rapidly closing with his enemy. His enemy had air superiority, and had just been reinforced by a force known to have many fighters of their own, so it was reasonable for him to fear either a renewed artillery attack or bombs, since his enemy had shown no honor and employed multiple traps. Given that, ordering his lines into close combat was a prudent tactical move, despite the fact that his forces almost surely out ranged their opponents, and given his reflective armor, being the only one not to close ranks would have made him an extremely tempting target, making that a poor tactical decision.
Alternatively, as I mentioned above, doing the long ranged dance is a safe way to win, but a slow way, and less than sure against an opponent who can break contact and fall back behind her own lines. Sometimes, you can play the long game. But, sometimes you need to win quickly. Sometimes, you have a chance to bring down the skillful and charismatic leader of a force on the edge of breaking before it can be fully reinforced, and you have to take it, even if at great risk. Had be brought Tara Campbell in her Hatchetman down, it might have broken the back of enemy resistance. This is not an unlikely outcome, if he can close and still manage to use his jets to stay more than 30 meters away, or if he can simply blast that Hatchet off, since the Hatchetman isn't a tough mech.
Lastly, it is noteworthy that Tara Campbell in her Hatchetman had just recently put Alex Hazen's beloved sibkin in the hospital. While I don't think it would be enough on its own to make him lose control, given the sound tactical reasons to close, it may have pushed him to close maybe just a bit faster and closer than he needed to.
And lastly, Tara's Hatchetman seems to be some sort of super Hatchetman, since it had stood up to Malvina's Shrike the day before. I don't know if its got some sort of super armor, or if its actually a Berserker masquerading as a Hatchetman, or if its Hatchet is actually an Elvish hatchet, made in Gondolin for the wars with the Orcs, but I don't think it was just a regular old Hatchetman.