So Megatron and Sentinal Prime had made a deal, and Megatron said they were supposed to rendezvous on Earth, but Sentinal was shot down before they could.
Assuming that the opening scene was the Ark attempting to retreat to Earth for that rendezvous meeting, why did the Decepticons shoot down the Ark then?
to break it down:
1. They had no idea he was on it. All they saw was an autobot ship retreating during a battle so they chased it.
2. Starscream had no idea sentinel was a traitor until he arrived in D.C which suggests that their deal was a secret.
3. After realizing that Sentinel had vanished and Optimus had taken his place as leader, the decepticons must have concluded that he was on the ship and tracked it down.