It has to do with how long after you initiate the move that you land, so that the attack animation has finished by the time you hit.
The only attack of pikachus that actually autocancles, I believe is fair, which doesn't have landing lag after the first couple frames.
I think its easiest to think about from standing at first, rather than from air.
For instance, Bair, if done from standing, you must do a full hop > bair in order for the attack to finish before landing, after which it will auto cancel.
Dair can autocancel if you input the attack fast enough after doing a SH.
Fair, nair, and uair can both autocancel from SH, but both nair and uair will have landing lag if you SH.
Its pretty simple to get the hang of with Pikachu, as everything but bair you can do from a SH.
But kinda get the hang of using the aerials from hops, and you'll have a good idea on the timing to use them while falling and not suffering landing lag.