Theory:

     It was at this point that my wife looked at me with an expression of wonder — not anger or exasperation, but just simple wonder — and said, “I can’t believe you do this for a living.”
 
     But I’m afraid it’s so. I always have catastrophes when I travel. Once on an aeroplane, I leaned over to tie a shoelace just at the moment someone in the seat ahead of me threw his seat back into full recline, and found myself pinned helplessly in the crash position. It was only by clawing the leg of the man sitting next to me that I managed to get myself freed.
Explanation:
 
While the narrator was panicking and being hysterical, his wife looked at him with an expression of pure wonder. She was neither angry nor irritated with him though she had all the reasons to be. But she was only astonished and bewildered, and she told him that she couldn't believe he did that every time, as if being clumsy was his calling or job.

The narrator admits that he could understand why his wife feels so. He says that he always ended up in disastrous situations when he travelled. After saying so, the narrator describes yet another incident that took place in his life. So you see, there is another story within the story. This feature is called embedded narrative (or a story within a story).

The incident which the narrator was about to narrate happened while he was travelling on an aeroplane. The narrator was settled on his seat when he discovered that his shoelaces had come undone. He did what anyone in the situation would have done: he leaned over to tie them.
 
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A man leans over to tie his shoelace
 
But what followed next could not be considered as something usual. While the narrator was leaning down to tie his shoelace, the passenger before him threw his seat back so that he could lie down.

Unfortunately, the narrator got stuck in the crash position between the floor and the seat's back. He lied helplessly for a while before he could manage to get the attention of his neighbour. He scratched the leg of the man sitting next to him and brought his plight into the latter's notice. In the end, thanks to the help from the neighbour, the narrator freed himself from the uncomfortable position.
 
Meanings of difficult words from the paragraphs:
 
S.No
Words
Meanings
1
ExasperationA feeling of intense irritation or annoyance
2
CatastrophesAn event causing great and usually sudden damage or suffering; a disaster
3
ReclineLean or lie back in a relaxed position with the back supported
4
Clawing
To scratch or tear something with the claws or the fingernails
Reference:
National Council of Educational Research and Training (2006). Moments. Accidental Tourist- Bill Bryson (pp. 56-59). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.