What wil cause the fuse for the seatbelts to blow/burn out?

What is your question?

I have just purchased a used Kia Sedona LX 2005. I test drove the vehicle on two different occasions about 2 weeks apart and no warning lights all looked good. After buying the vehicle the Airbag/seatbelt light came on. Checking the interior fuse box I found the seatbelt fuse burned out. I changed it but it instantly burned out again. How would I determine where or what is causing the fuse to burn out, I assume a short circuit, broken or pealed wire or bad sensor. I am a large man perhaps 270lbs and I did adjust the driver seat including lumbar support and seat cushion levels. I have looked under seat to visible check connections for seatbelt sensors and wires and all generally look good.