Wouldn't you be shocked by the Gundam you had "secured" exploding to expose a smaller mobile suit underneath? Tieria took that opportunity, grabbed the shoulder guns from them again (recall that the parts exploding outward hit Soma and the other HRL suits) and blasted the hell out of them.

They had other things to worry about, and the scene doesn't show well how quickly that all happened. The animators were too busy doing multiple angles of the girly gundam.