Yīqǐ páshān ma (一起爬山吗)? Why does this sentence make most Chinese people panic?

Suppose you see the following scene transpire between two friends: Friend A: yīqǐ qù páshān ma? 一起去爬山吗? Want to climb a mountain?

Friend B: (lüè dài jīngkǒng de) bùle bùle (略带惊恐的)不了不了 (Slightly frightened) No no no…

If you’re not from China, you might be confused …