meaning "what is your reasoning?"

It is a slang phrase that is often used with a little intentional arrogance.

In such cases, the phrase implies that whatever was just said is ludicrous. Thus, the phrase can be quite sarcastic.

This nature of the phrase was a prominent lyric in the song "Cameras" by Drake.

Similar phrases could be:

1) Hell you talkin' bout? (What the hell are you talking about?)

2) Wtf? (What the fuck?)

3) Excuse me?

HOWEVER, the phrase can also be a playful way to genuinely ask "What do you mean?" or state "I don't get it."

The safeguarding of Earth and other worlds from biological cross-contamination (i.e. billionaires with too much time on their hands).

Also known as “planetary protection.” Planetary protection / quarantine “reflects both the unknown nature of the space environment and the desire of the scientific community to preserve the pristine nature of celestial bodies until they can be studied in detail.”

There are two types of interplanetary contamination. Forward contamination is the transfer of viable organisms from Earth to another celestial body. Back contamination is the transfer of extraterrestrial organisms, if such exist, back to the Earth's biosphere.