In Quincy, MA two patrons at a Mc'Donald's chucked coins and safety cones at three cashiers after receiving small fries instead of the large size they ordered. This isn't supposed to be a funny story but I laughed reading the whole article, which you can read here.
It's just such an extreme and INSANE reaction for such a small infraction, one where a simple, "Um, you gave me the wrong size fries," should fix the problem. This misalignment makes a moment of rage comical. What's even funnier is it was the second fast food altercation in the area within a four-day span. What is wrong with people though?! Seriously.
An important aspect for believablitity in fiction is stimulus and response. In order for your characters to be believable, you must provide appropriate stimuli and catalysts for story directions and the decisions and choices characters make. Keeping stimulus and response in mind, you can get your character to do anything you desire. You just need to brainstorm and come up with an appropriate stimulus.
In the movie Falling Down, Michael Douglas's character, Bill Foster, gets pushed over the edge after a hellacious morning where everything possible has gone wrong. He pulls a gun on a cashier at a fast food restaurant because they stopped serving breakfast. Such a small infraction again, but now we have a reason for the postal moment. Because the tension/stilumus was built up beforehand, it still looks as crazy as the Quincy nuts, but we can understand what made Bill snap. He's a believable and identifiable anti-hero and we follow along.
Make sure you provide the nudges your story needs to make sense and not be laughable.
~ Signing off and sending out cyber hugs.