This is a bug that has persisted for years, but nobody has really done a deep-dive until recently about why this has occurred.

Basically, airblasting a projectile attempts to redirect that projectile towards wherever you're currently aiming. This erroneously includes other projectiles, including the projectile you're trying to airblast. So, aiming directly at the projectile you're trying to airblast will make the game think that the direction you're trying to airblast the projectile in is towards yourself, since your crosshair collides with the very side of the projectile's bounding box, which will always be closer to you than the center of the projectile, leading to the projectile to be airblasted towards you.

This video visually shows the bug in action and what causes it to occur.

The ideal way to fix this would be to ignore projectiles themselves and ensure that it only considers map geometry and enemy players when trying to decide where to redirect the projectile.