Why do people use question marks when they aren't asking a question?
Some people may use question marks to ask a rhetorical question. A rhetorical question is a question which is not meant for answering. For example "Do you know why people put animals in cages?!!!" And then right after, the person normally puts the answer right after the "question" themselves.
A couple of thoughts...
Suppose you start writing (or speaking) a statement, but by the end of it are not sure that:
The reader will agree.
What you say is necessarily true.
Then you might end with a question mark?
The head of department was trying to protect us?
Using a question mark at the end of a statement does turn it into a question - in a lazy sort of way. It could be thought of as an efficient way of making an opinionated or tentative statement and enquiring of the receiver as to whether they agree with you.
Whether this is always grammatically correct I am not sure, but it does seem to be a popular practice.
They could be unsure of themselves.
For example, if a teacher asks a question and a student hesitantly answers, it might sound as though the student is asking the teacher if the answer is correct:
"What's the capital of Australia?"
"What's the definition of lecherous?"