Well, to my mind, NO historian has really explained what the hell he was doing at San Jacinto, with the Texians right out in the open, advancing on his camp, with two cannons clearly in the open, without really calling his troops to arms.
He didn't get the alert until it was far too late. Everyone in the Mexican camp was dead tired from the weeks of force-marching.
The other thing that you should consider is that the previous day, he'd force-marched his troops all day, set up camp for the night, discovered that the Texians weren't far away, and had his troops erect hasty fortifications and stand-to to repel a night attack.
The night attack never came, his troops were exhausted, they had to rest. He posted additional sentries, figured that a "peasant rabble" could never successfully attack his force during the day, and took a nap.
And Sam Houston, the failure of failures, who had been accused of running away from everything and everyone, brought his outnumbered but enraged troops in and crushed Santa Anna's army in 18 minutes.