It’s really not that wild of a theory.

• The Australian government of the time was very left wing, doing things like creating universal healthcare and education.
• Whitlam was threatening to shut down Pine Gap, the joint Australian-US spy station in the middle of Australia, and a key location for the CIA’s information gathering efforts in Asia.
• The Governor-General of the time, John Kerr, had known ties to known CIA-backed organisations.
• The US in the lead-up to the Dismissal appointed as its ambassador to Australia a man who has known connections to a coup in Indonesia, and he was also the ambassador to South Korea during a coup there.
• Whitlam even threatened to reveal the identities of CIA agents in Australia.
• A US military contractor has spoken out and said that the CIA internally referred to the Governor General as "our man Kerr".

The US had many clear motives for wanting this. They had the means thanks to Kerr. And they were afforded the opportunity by the Senate deadlock. There are certainly a few reasons to say otherwise, but at the very least it’s impossible to dismiss the theory as crackpot.