What is the main cause of transformer when shutdown?

It could be because of overloading, or short circuit breaker opening

emergency shutdown and system breakdown

Repairs to a power transformer resulted in the shutdown of one of Exelon Generation's reactors at its Quad-Cities Generating Station in Cordova, Illinois.

to avoid from short circuiting 

i am very hard worker and honest person.

Repairs to a power transformer resulted in the shutdown

Save your work against losting , and Save your Infrastructure

overloading maintaince work

overload and max probably short circuted

In order to achieve the minimum value and goodwill of the firm.

Due to the luck of Service or luck of Exprience or luck of finance