What was happening ...?
Use the past continuous to express what was occurring at a precise moment in the past. The past continuous is often used together with the simple past to express an interrupted action. The action which was interrupted is expressed using the past continuous, while the action which caused the interruption takes the past simple.
Subject + to be + verb + ing + objectsTom was speaking off the record when he admitted to his crimes.
Subject + to be + not + verb + ing + objectsI wasn't writing any copy when I started on-the-job training.
Wh? + to be + subject + verb + ing?What was happening on the ground when the project began?