Murphy was a Bastard

I was mowing our 5 acre block,
Just yesterday,
When Murphy's Law came visiting,
In a rather big way.
We have a small tractor,
For without it mowing is a chore,
However, I quite like the solitude,
As I fly around full bore.
But I was in the farthest corner,
And no matter how I tried,
The tractor gave a few strangled coughs,
Then gurgled badly, then died.
I tried to restart the machine,
All to no avail,
I tried all the bush mechanic tricks,
But death did still prevail.
I trudged back to the house,
To get every single tool,
I trudged back to the tractor,
But I still looked like a fool.
There was nothing left but to strain,
Shoulder to the wheel,
Pushing the heavy mongrel tractor,
Face set hard as steel.
Over hummock and undulation,
Through the long grass and the short,
It took me over an hour,
But success and I can report.
My question is simply this,
"Why must the worst happen precisely,
At exactly the worst moment?"
Can you answer concisely?
At least when the mechanic came,
It wasn't easily repaired,
For if it was something silly on my part,
My embarrassment couldn't be compared.