Bow Shock was their only chance.
The undead army was approaching, the surprise attack by the necromancer giving them no time. The Princesses were too far away to get there soon enough. Ponies would die.
He couldn't let that happen.
So Bow Shock took the one route he had. The one weapon at their disposal that could end this with only one death.

All he needed was a few moments in free fall, if he didn't get a boost, and even the famous Rainbow Dash would be jealous. Too bad he wouldn't be around to see her face after this.