Babies often have involuntarily moments. This is normal as the muscle control is still developing.
When babies are sleeping, these movements can actually cause the baby to wake them selves, either from the jolt of their own movements, or when they hit themselves in the face.
To help with their sleep, to keep them undisturbed, simply swaddle the babies arms. This helps keep the babies warm, stops involuntarily arm movements, and gives the baby the safe reassurance, similar to when the were in the womb.
Information in this blog is from my own personal experience / observation and only for my own reference. If you find it useful, then great, buy it shouldn't be considered "gospel", and your own experience may and will differ as each baby / situation is different.