Back in melee if you did a wavedash in front of a lvl 9 CPU, 9 times out of 10 they would grab. The reason is they were reading your inputs and assuming you were shielding because the game wasn't built to understand a wavedash, but the button input is the shield. Smash Ultimate is no different.
When an AI fighter is in a neutral stance (no actions other then walking), they will typically react to your button presses appropriately. Shield and it will grab. Smash and it will do the fastest move it can to avoid it (jab interrupt, perfect shield, etc). The AI is also annoying good at cross-up (landing behind you when you think it will land in front of you). The AI also has an excellent sense of spacing, knowing when it can hit you but you can't hit it particularly if you do an attack that will definitely miss it. Basically, if you play very aggressive rush down it will counter almost everything you do. Characters with more reach, such as Swordsmen, Bowser's F-smash, Pichu's Skull Bash, etc, are particularly frustrating to deal with because of the AI's perfect timing/spacing with them.
The AI is actually really bad at handling attacks started slightly outside it's shield bubble and hit boxes that persist for an extended period. This is why character's like Ike are so easy to land a smash with, but character's like Pikachu feel almost impossible. Playing very campy works wonders, particularly on some of the awful stages like Peach's caste. Make the AI come to you, try to bait it to do an attack, and punish. If you play keep away, it has a tendency to randomly smash attack, leaving itself vulnerable. With every option being available out of dash, most characters should be fast enough to get in there and at least dash attack or grab.
The AI is also really bad about protecting itself through platforms (like on battlefield) so if you can hit through them, you are much more likely to get a hit without being punished.
BONUS TIP:
Using the 1 stock/reset to trick to unlock everyone?
If you exit the game before you lose the match. It doesn't save that you fought that challenger. They will show up again next time you complete a vs match.
This also works for the door rematches in the Games & More section.