Impossibly Mundane Explanation

Bob is acting unusually. Maybe he knows something he shouldn't or is spouting highly advanced facts about nuclear physics in Latin. The leading theory is extraordinary — maybe he's possessed, or maybe his Evil Twin has taken his place. Someone will propose a mundane alternative like, "He read a book about it". Everyone rejects it out of hand because it would be out of character for Bob, and they return to the extraordinary theory.

This mostly occurs in Speculative Fiction. It serves to reinforce our characters' comfort with the extraordinary while simultaneously reinforcing characterization. The rejection will frequently be nonverbal, consisting instead of "Are you crazy?" looks.

Bonus points if said mundane explanation actually is the explanation.

Note that not every case of characters choosing an extraordinary explanation over a mundane one is this trope. This is only if the mundane explanation is rejected out of hand because it would be completely out of character for said person.

Examples of Impossibly Mundane Explanation include:

Anime and Manga

Umineko no Naku Koro ni: Cardboard boxes and Small Bombs.
- Trying to find a Possible Mundane Solution seems to be how Battler can win.

Literature

Dirk Gently does this to some extent in The Long Dark Tea Time of the Soul. When he meets a girl who constantly recites the previous day's stock quotes (in real time, just with a 24-hour delay), he rejects the assumption that she's just memorizing them somehow (after all, the information is out there!) in favor of some more mystical explanation, because nobody would ever go to that much trouble. It's a little different since he's arguing on the basis of general human nature, not specific character, but the principle is the same. Dirk sums this up by reversing Sherlock Holmes' usual maxim: Eliminate the improbable, and whatever remains, however impossible, must be the truth.
In Stephen King's The Mist, one of the grocery store's occupants poo-poos the idea that something unnatural is loose in the fog, claiming the locals set up the tentacles as a prank. A witness to the maybe-squid-thing's attack sarcastically replies that, yes, they spend thousands of dollars to buy a giant squid from an aquarium and truck it to town in a tank, just to play a prank on the man.

Live Action TV

The Stargate SG-1 episode "Window of Opportunity": Jack has prior knowledge of a briefing Carter is giving, and claims that he's remembering things from the future. Carter suggests, "Maybe he read my report?". Daniel gives her a look and repeats, "Maybe he read your report?" as if it was the most ludicrous suggestion. Everyone else (O'Neill included) seems to agree.
On Buffy the Vampire Slayer, the gang is attempting to contact Buffy.

Xander: Well, she didn't go home. I let the phone ring a few hundred times before I remembered her mom is out of town.
Giles: Well, maybe Buffy unplugged the phone.
Xander: No, it's a statistical impossibility for a 16-year-old girl to unplug her phone.
Willow: *nods*.

- There's also the time that Xander was possessed by a hyena. Buffy quickly figured out he was possessed while Giles believed he was just being a teenage boy. Knowing Giles' teen years, this assumption makes a bit more sense.

Web Comics

Freefall has some fun with this. Apparently, Florence is a vampire. Either that, or she really swam in the cold water a bit too long.
El Goonish Shive has a borderline example.