「how come」はどういう意味ですか?

This guy I saw on the train the other day cracked me up.

• crack up - 大笑いさせる、爆笑｛ばくしょう｝させる、ゲラゲラ笑いだす (definition from Eijiro on the Web)

His shirt says:

• LAND LORD EXTREME

A landlord is a person who rents their property to others.

• 大屋 (おおや) (n) landlord; landlady; (P); (definition from Edict)

In English-speaking countries, we often use "extreme" to describe something exciting or dangerous, like "extreme sports".

• extreme sports - 極限スポーツ、アドレナリンを極端｛きょくたん｝に増加｛ぞうか｝させるような新種｛しんしゅ｝のスポーツ◆バンジー・ジャンプなど

What I want to know is how come somebody decided that there was anything extreme about being a landlord?

"How come" is a casual conversational way to say "why". Notice, though, that the usage is a bit different. In particular, notice the position of the subject and the verb.

• how come どうして、なぜ◆【同】why
  

○ Why did you do that? (standard English)
× How come did you do that?
○ How come you did that? (casual spoken English)

○ Why are you standing there?
× How come are you standing there?
○ How come you are standing there?

Even more than the reason someone made such a sweatshirt, how come he decided to buy it? I'm warning you: don't wear anything with English writing unless you are sure about what it means.