What causes the incident to take place outside of Alex's house in Chapter 12?

The incident that occurs outside of Alex's house in Chapter 12 is due to an investigation by Milo. Milo has been watching a man named Ernie Radovic. When Milo sees Ernie approach Alex's house, Milo stops him. Ernie Radovic is a supposed crazy man who is Digby's bodyguard. Due to Digby's death, Milo begins investigating Ernie.