During a discussion, people were criticizing the teen singer Justin Bieber of his poor style of dress on one of his photos wearing an outfit that included a drop crotch pants. Someone then mentioned about his roomate who wears the same style of pants, he said:
My roomie has like 4 pairs of rick owens drop crotches.
Another person replied:
That’s pretty ballin but I just can’t get behind the drop crotch look.
What does get behind mean? Does it mean giving support to or approval of somebody/something?
I wouldn’t like to say for sure it’s the “origin”, but OP’s usage strikes me as a shortened version of…
get behind the mule (and push him, to help make progress in difficult terrain)
…which itself is primarily a modern idiom (popularised by Tom Waits among others) that alludes back to…
put your shoulder to the wheel (help the mules/horses move the wagon forward, by pushing)
That’s to say, the basic meaning of get behind is support, as OP surmises. But in practice people often say they can’t get behind [an idea, or a proposed course of action] when what they mean is they don’t accept it. And sometimes (perhaps by association with get = grasp), when they don’t understand it.