He exhibited the same arrogance that Capt Fetterman did when he bragged before being wiped out.

He didn't believe his scouts when they told him "If you go down there you will all die."

He divided his forces. He was out gunned. The Indians fired 13 rounds to one for every military round fired.

As stated above, when he dismounted his troops, he gave up the only advantage he had. However, his troops and horses were completely exhausted and probably couldn't have mounted an adequate charge to overcome the best light cavalry in the world.

He was in deep doo-doo from the outset.

When he dismounted his troops he was simply overwhelmed by superior weapons and firepower.