However, it seems you wanted to create a surprise for the reader.
"What? Only 5%? That's terrible!" ... and then... "Oh! I see! 95% before they ever graduate. Well, that's different! What a wonderful program that must be."
This is lost when you join them. Joined, it sounds like the answer to the question "Why are only 5% of your students placed after graduation?" "Only 5% are placed after graduation because the other 95% are placed before graduation."
You can avoid the informality of starting with "Because" by replacing it with a dummy subject: That's because ...
(And YL, I do appreciate your confidence, but I certainly have missed things in the past! However, given the length of this thread, I would have come back to correct by error well before now if I felt that the verb had been incorrect. I agree with you on the verb - 5% of the students are..., 95% of the students are...)