It's the way the sentence is constructed. :3

Like... "The girl built the house" is active voice, while "The house was built by the girl" is passive voice. People, especially teachers, usually think that passive voice sounds weaker than active voice, so they tend to take points off when they find it. An easy way of telling if a sentence is passive instead of active is if the sentence has a form of the verb "be" and another verb, it's probably passive voice. :3