Theory:

     The zip on the bag was jammed. So I pulled on it and yanked at it, with grunts and frowns and increasing consternation. I kept this up for some minutes but it wouldn’t budge, so I pulled harder and harder, with more grunts. Well, you can guess what happened. Abruptly the zip gave way. The side of the bag flew open and everything within — newspaper cuttings and other loose papers, a 14- ounce tin of pipe tobacco, magazines, passport, English money, film — was extravagantly ejected over an area about the size of a tennis court.
Explanation:
 
The narrator proceeds to describe the incident.
 
So this is what had happened. The narrator tried to open the bag's zip. However, as luck would have it, the zip was stuck. He tried pulling it open, but nothing happened. He pulled it harder, and with more force and focus.
 
He slowly started to feel the pressure on him. People were waiting, and they were observing him. They were probably judging him too.
 
So the narrator yanked at the zip harder than before. He was so frustrated and anxious that he did it with increasing grunts and frowns. He kept this up for some time with no positive outcome. Finally, he pulled it harder with all the strength he could summon.
 
gui-1294417_960_720.png
The narrator tried pulling the zip that was stuck
 
It is not hard to imagine what would have happened next. Yes, the zip came undone without any warning. The narrator lost his control over the bag and dropped all its contents on the floor.
 
The narrator's bag was filled with things such as newspaper cuttings, some loose papers, a 14- ounce (i.e., approximately 397 grams) tin of tobacco for his pipe, magazines, passport, English currency, and filmstrip.
 
filmstrip-1174228_960_720.png
A filmstrip
 
The real problem here was not the fact that the narrator dropped the things on the floor, but rather he dropped them all over the place. As he likes to emphasise, "everything was extravagantly ejected over an area about the size of a tennis court".
 
While the phrase "extravagantly ejected" suggests the speed at which the contents flew out from the bag, the phrase "the size of a tennis court" indicates that the things were spread across a vast area of land. It also implies that the narrator was going to have a harder time finding all his belongings, despite the fact that the contents came out quite abruptly and quickly.
 
Meanings of difficult words from the paragraph:
  
S.No
Words
Meanings
1
JammedUnable to move or work
2
YankedPull with a jerk
3
GruntsTo make a low inarticulate sound, typically to express effort
4
Consternation
A feeling of anxiety or dismay, typically at something unexpected
5
BudgeMake or cause to make the slightest movement
6
Abruptly
Suddenly and unexpectedly
7
Ounce
A unit of weight, approximately 28 grams
8
ExtravagantlyIn an excessive or elaborate way
9
EjectedForce or throw something out in a violent or sudden way
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.