Chapter 1: Protect
Ezra knew Maul was up to something when he began to coax him away from Kanan. He may have gained a little of his trust, but Ezra was no fool. Living on the streets for seven years taught him many lessons on how to survive, and one of those lessons was how to spot people who wanted to take advantage of you. Ezra encountered many shady people in his life, all of them talented in the abilities of manipulation and lying. He had to learn the hard way of seeing past the false pleasantries and promises people always made to not get extorted out of a deal.
Even without the Force Ezra knew when he was being manipulated, when he was being charmed to do what the other person wanted. Maul had a way with words. He wasn't the typical con artists Ezra had encountered, but he shared some of their abilities nonetheless. Maul didn't lie, which made you want to trust him, but when he did speak, every word seemed to be coded in honey for prey to stick on.
Ezra was familiar with guys like these. And he was no prey.
So when Maul asked him to go on ahead, he refused. Giving him a quick "it's better if we stick together" he began moving closer to Kanan. Knowing if he left him alone, Maul would go after him.
It seemed as if Maul was not pleased with Ezra's answer, and for a moment, he let his defenses slip. At that moment Ezra felt the intensity of his hatred, his anger, his rage, his intent, and saw the exact moment Maul redirected it to his master.
Ezra acted before he even saw the red of Maul's blade.
In hindsight, reaching for his lightsaber would have been a better move than using his body as a shield, but Ezra knew there was no time. With force screaming "danger" and his instincts pushing him to run, Ezra did the only thing he could do.
Protect his master.