He was filming the plane, and stopped filming as soon as it was out of sight. Then there was a big explosion, so he started filming again.
Think trainspotters, planefags, etc.
If it was a terrorist lacky filming a planned attack they would have found a better vantage point to film from, and used a better camera.
<And YES, I ASSUMED HIS GENDER. Come one man, how many female planefags do you know?