From the later Toaru loredumps, its because Aleister's Espers are built off an offshoot of Telesmic theory, which involves constructing your body, mind and soul into a temple to house your personal Angel you cultivate within, which will let you grant yourself your own miracles, which reacts...badly with other forms of magic, which invokes external powers and mythos by twisting yourself to fit them, contaminating your Temple.
Which causes the power in your Temple to explode. It doesn't matter whether your Angel is powerful or not, once you've entered the Esper program and built your temple, contamination causes explosion inside your Temple.
And since your Temple is your body...
...splat.
That said, its not impossible. We have plenty of canon examples:
-Tsuchimikado's Esper Power is Auto Rebirth, so when he uses magic, it explodes his body and his body rebuilds after that. The hard part is to keep his concentration and finish casting before the explosion breaks the spell.
-Umidori does it at one point deliberately. Because she's heavily cyberized, she simply autohypnotized herself so that she considered a large structure to be part of her own body, allowing the explosion to destroy that instead of her body.
-Accelerator does it, I don't think it was explained properly, but my personal theory was that he switched the order around, because he just interpreted magic as a subset of his Vector Control power and could thus use magic indirectly without warping his self.
-Aqua of the Back isn't an esper, but as all four of God's Right Seat are Telesmic style magicians, they have the same restrictions on using off-style magic, but Aqua's style is that he can use every other style.
