Hmm, I don't think I knew this. And I'm not sure if I find it very satisfying.
Did this happen on more albums?
It was a rather known single instance, just like Face in the Sand double kick. Seems Nicko was open to some new stuff in this era.
"drums on album being sample replaced" is a very raw statement that drops some context. I'm sure Lego knows all about it - MIDI triggers are used on the drum to provide digital information about every drum hit, they are small microphones that automatically map sounds to MIDI, placed very near the drum skin or cymbal but they don't impede the instrument, thus are used with normal audio microphones in conjunction normally. This allows drummers to have two things at the same time in the recording - acoustic drum and electronic drum track perfectly aligned.
McBrain used this, and Shirley put patches of his own drum in the sampler/drum machine that renders the MIDI data. The end track is mix between real drum sound and digital sampling of the very drumkit.
Why do this for Maiden?
Well when you have album where instruments are compressed to the bone, if you use standard dynamics of the drumkit it's possible that certain drum hits are not going to be big enough to fit the sound. With this tech you can use sound of the maximum snare hit whenever you want, at any volume you want, because sampler is involved. Even if you hit the snare normally, the sampler can play the maximum hit sample at a lower volume. Because real drums have different sound depending on how hard you hit them. So what they did for guitars they did for drums too, but they couldn't use normal compression because drums behave differently.
Btw a lot of death metal technical drummers use MIDI triggers for live kicks to work around the tuning issues between two bass drums.
P.S. compare Dream of Mirrors album vs RiR. 4 minutes in, over the verses where drums have no bass kicks. Although Bruce is singing on the lower level, Harris is not plonking the bass hard, guitars are acoustic, on album snare sounds like it's hit like a truck constantly, but the drums are not volume loud, they don't jump out of the silentish dynamics. RiR, you can hear how the snare is a bit different sounding, still not volume loud, but still round and full sounding. Now go to RiR TV and you can hear the real drum sound. No loud play no big drum sound, they sound completely standard in this section. So there you go - MIDI vs analog compressor vs dry track.