I'm guessing because he ****ed up, realised he'd ****ed up, grabbed the defender's shirt and fell over.
He didn't have the ball in front of him though, he mis-judged the bounce so it was behind him, that's when he grabbed the defenders shirt and fell over. did you not have the replay shown on what you're watching?
Fck knows what you're watching. They've shown it again. The defender went through him and clipped his leg. Armstrong is in control it's a blatant pen. The rest is immaterial.