The most natural translation would be "What is the root cause of this behavior?" "Source of behavior" sounds odd in English, and I would argue that מקור and שורש are pretty similar in meaning.
I think simply "cause of this behavior" would do, if the woRd will take that translation. I do, though, see the term "source of behavior," especially in the sorts of pop psychology stories that appear in the press. A Google search turns up lots of contemporary examples.
I would expect a של after מקור (to go with the "of" in English). Why isn't there one (or could there be)? Is it because this is a construct? Could you say, instead, המקור של ההתנהגות ?.
Yes. They can be used interchangeably. Historically the construct state was preferred but now, except in certain constructs, של is preferred in colloquial hebrew.
It depends on the ניקוד (or vowel). Makor means source and should be המקור של הבעיה. However, the word used here is mekor which means "the source of". (This is called a סמיכות, which is similar to a compound word but is written in 2 word form. In the first word in a סמיכות the ניקוד of the first word is changed. E.g. beit sefer - and not bait sefer.)
It is in the construct form ("source of", "origin of"), so it becomes "m'kor"
I think it should be "The source of THIS behavior" and not "THAT behavior" because in this context it seems like something closer to home like THIS
Not quite, מקור is "source", "origin". "Causes" is סיבות, גורמים. When talking about behaviour I suppose we would mean the same thing. But we could say מקור המאכל הזה for example, or מקור המנהג הזה.