As a teenager, I sat with my family in the Mormon church. When they called us to pray, I would pray according to how they taught me–hands folded across my chest. Even then, I considered that peculiar.
A child in rebellion crosses her arms over her chest.
Someone seeking a holy God mimics those saints before her and bows her head, goes prostrate, or kneels–all signs of respect and honor to God.
Bear in mind, I am not saying we should make sure by appearance we are praying to seek the praise of men. You can pray while driving to work, working on a computer, taking a walk, etc. Prayer is conversation with Him. You can even pray with your eyes open.
Mormonism is rebellion against God so, as a believer now, I can see how such a simple gesture is symbolically rebellion, too, like a child crossing her arms over her chest and saying, “I want it my way.”