Fast players against slow defenders

That's called auto defending, it's when someone doesn't touch the joystick and lets the autoplay do the defending for them. A lot of people abuse it because not only it follows the attacker perfectly even when changing directions, pace and joystick dribbling are also useless against it. It has one drawback which is giving too much space to shoot but it can easily be removed just by timing the standing tackle whenever the attacker gets too close.

The crabwalk that you saw is one of auto defending perks.

Again it's a mechanic that people have abused for years and sadly EA did nothing about it.