The term 'Beginner's Luck' when applied to fighting games pretty much applies to what @
Nah
linked. Fighting games have always been about reading the player and not fighting the character in higher levels of play, and when the opponent has no base knowledge to apply to the fight they'll start experimenting. You really can't guess what experiments they'll be doing, which is why they'll catch you off guard. As they get better, they start developing patterns that veterans can read and start learning just like the rest of us.
It can be frustrating, but it is how it is. It's why you can suddenly be beaten by a 6 year old who has only the base knowledge of how games work, as they are the closest a human can get to a true random generator.