Translation:I do not know why those jobs are done at night.
"I do not know why those works are done at night." Spanish may use the plural "trabajos" here, but in this context, English uses the singular. "this work" should be the preferred translation. I understand the principle of testing recognition of plurals versus singulars, but the goal should also be to understand how language is used differently in the different languages.
"estar" is used to describe someone's/something's state or condition, while "ser" is used to describe action and it is almost always used with "por". For example, "Las pizzas son cocinadas (action) por mi mamá, pero ella está enferma. Por eso, las pizzas no están cocinadas (state/condition).
The context of the sentence makes "made" a really unlikely translation.
I don't know why these JOBS are done at night. Jobs are done, not made.
I don't know why these THINGS are done/made at night - if this sentence had used a different noun, like "things" - either "done" or "made" would make sense.
Have a read of this too for some tips: http://forum.wordreference.com/showthread.php?t=9572