I'll say it isn't because your stirrups are too long, since I keep mine to where I can only get a finger (maybe 2 tops) between my crotch and the saddle. I don't jump, so I don't have the need to get as far out:
Some things to think about - or maybe ask your instructor about -
1 - What are your stirrups/boots made of? I need rubber on at least one of them. Leather sole boots on rubber pad, or rubber soled boots on bare metal.
2 - Where exactly is your foot in the stirrup? If my foot is a little too far back, then I'll subconsciously point my toe to 'hold them' on my foot. Changing the position of the stirrup by as little as a half inch, or angling it differently, can stop me from doing this.
3 - Watch pinching with the knee. I often need to make a conscious, thought-out effort to spread my knees. As the years go by, it is becoming less of a problem for me, but pinching with my knees was a HUGE problem when I started. As a variation, I am also capable of pinching with my thighs, pushing me up out of the saddle slightly and making me point my toes to 'catch' my stirrups.
4 - Bracing with the leg will cause bouncing, and bouncing tends to make you point your toes to keep the stirrups.
Those are some of the problems I've had over the last 5 years. I'm sure there are others. Good luck, and talk to your instructor
Edit to add 5: If my balance gets too forward, I'll also point my toes.