I don't know if this is the case in Castlevania 3, but in Cv1 if you were to jump from a big height and pressed attack just as you were to land you'd whip while crouching. I haven't played in a while so I don't know if the same happens if you use subweapons, or if this carried over to Castlevania 3.
But hey, just throwing suggestions.
Suggestions are good! But this one doesn't appear to be the case. Performing any attack in air makes you go to the standing attacks, although the moment you land it goes into the ducking sprite, which creates a visual glitch where Trevor's hand is disconnected from the whip. If this really was a thing in CV1, then they "fixed" it in CV3. But none the less I appreciate the suggestion.
I found the sprite animation data eventually after trying your suggestion. I think this sprite was from a removed feature. Maybe Sypha was supposed to use spells by default? Anyway, I first found the actual reference to the sprite. These are the animation tables for spell sprites:
34082C10FF ; standing spell (2 frames)
36082E10FF ; ducking spell (2 frames)
380830100A01FF ; stair down spell (3 frames)
3A0832100C01FF ; stair up spell (3 frames)
I just needed to figure out how to access it. So I tried tracing the references to that data and it's inside the Item/Spell Use sprite change subroutine. Sweetness! So I traced that and found the next set of data that it used to decide which animation to use. It's basically a map of each spell-casting state Sypha can be in:
20 00 ; standing spell state, use animation #00
22 00 ; jumping spell state, use animation #00
1C 02 ; ducking attack state, use animation #02
24 04 ; stair down spell state, use animation #04
24 06 ; stair up spell state, use animation #06
I figured I must have messed up earlier when I studied Sypha's code and double-checked if state #1C really was the ducking
attack state. Yup, it is. So the sprite itself is actually mapped to Sypha's ducking attack, but her attack states use a different set of animations. I guess the sprite can't be accessed normally after all.
Oh well, it's the willingness to help that counts.