As the bullet slammed onto the tip of the missile, it caused the volatile chemical within the bullet point to ignite. The bullet's explosion didn't exactly caused the missile to explode but rather forced a jerk in the plane's movements since it only hit the guidance system when it met bullet tip to missile tip.
But this already made the plane move downwards because the pilot was unsure what was happening and forced him to auto react by pulling his stick downwards. This sudden movement complimented the second bullet from Tellie's double tapping going through the hole that the first bullet had made by exploding the missile guidance system causing a series of chain explosions.
The second bullet penetrated further through the missile guidance system before hitting the portion where the missile's warhead had been placed, causing it to explode and destroying the internal portions of the plane, forcing it to go into flames.
