What is the connection between The Avengers 1 and 2? Why did The Avengers 2 start with the Avengers attacking Hydra?

Before Reunion 2 was Captain America 2, which involved Hydra.

In the reunion of 1, the black widow closed the passage with Loki's scepter, and the natural scepter remained in the shield. Hydra of Team 2 of the United States "undercover" in SHIELD and got it, so at the beginning of the reunion of Team 2, they attacked the base of Hydra and regained the scepter.