This idiom or expression means:

Out of control.
An irresponsible and reckless (therefore potentially dangerous) person.

I like him and I really wish I could nominate him for the job but, let’s face it, he’s a loose cannon!

This idiom or expression means:

Causing or helping a person (or an idea, situation, etc.,) to get out of control.

When I encouraged my ex-wife to become a lawyer, I didn’t know I was creating a monster. Now she’s using her legal mind to drive me out of business!

This idiom or expression means:

Getting out of control.

A. Did you tell the kids to go out in the backyard?
B. Yeah, they were running around in the house and things were getting out hand!!

This idiom or expression-like sentence means:

The system is about to break down.
The situation is getting out of control.

Q. Do you think the government is losing control of the economy?
A. Definitely, to the point that the wheels are coming off the wagon!

Also:
Wheels falling off the wagon.