It's a common question. The answer is that it depends on the situation. "What brings you here?" is the same as "What do you want here?", "Why are you here?" or "What is your purpose for being here?". You might be the first stranger in a long time in a small town and a local person asks you "What brings you here?" "What brought you here?" is more like "Which circumstances happened in order for you to come here?". If you have ended up in prison or in a hospital bed, someone might ask you "What brought you here?" and you would answer "I broke the law" or "A traffic accident".

Hello! What brings you here?