Bernard was part of the Tailies because he was in the bathroom when the turbulence started, Cindy the flight attendant informed the passengers to buckle up because of the turbulence. As Bernard was making his way back to his seat the plane jolted again really bad and he decided to buckle up in the nearest seat as would anyone do when someone experiences that much turbulence. It is basic human instinct to protect oneself when facing great danger and the only way he was able to protect himself was strapping into the nearest seat available.