The plane, a USAF AWACS (Airborne Warning and Control System) aircraft was destroyed by the heat of the alien ship's entry into the atmosphere.
The destruction of this plane may be unintentional, but flying at speed into someone's airspace is de facto an act of aggression, especially if you plan to wipe them out.
From the shooting script;
INT. AWAC - SAME The Pilot squints as he tries to see through the clouds.
PILOT : Wait a minute, it may be clearing.
Suddenly the clouds part before us and we're face to face with a WALL
INT. AWAC - SAME The speaker phone cracks and distorts.
PILOT : Jesus God! The sky's on fire!
EXT. AWAC AND PHENOMENON - SAME The AWAC attempts to climb sharply as we get our first real look at the atmospheric phenomenon. Majestic
and monstrous. The AWAC is not going to be able to make it.
Quickly it is ENGULFED in the flames.
and from the official novelisation
Growing louder, a sonic disturbance, the cracking hiss of static
interference, growled over the speakerphone. Then, just as the AWACS
broke free of the clouds, the pilot’s voice screamed over the noise,
“Jesus God! The sky’s on fire!”
In front of him was a solid wall of flame five miles high and twenty
miles long, a majestic and fearsome sight. Roughly disk shaped, it was
shedding altitude, dropping down right on top of him. The pilot jerked
back on the controls, forcing the plane into a steep climb. But when
they came too close to the fireball, the plane suddenly shattered like
a lightbulb crashing against an anvil.