>>76543085
Preacher curls load the lengthened position more. The lengthened position is where there is reduced active tension and increased passive tension. You don't generate nearly as much mechanical tension from contracting your muscles and most of the tension is from your muscles being at full length and having to support the load. Because of this the muscle is like a rope holding on to a weight. It is under a lot of shearing force and the muscles being much weaker than tendons tends to tear under too much load in that position.

BTW, it's not the tendon that tears. It's usually the muscle tissue that tears in cases like that. Tendons generally are much sturdier than muscles. Too avoid the problem just stop short of full extension. Say at 30 degrees of flexion to keep your muscles in a position where you still have a lot of active tension while at most a manageable amount of passive tension. Also don't 1RM preacher curls. If you want to 1RM do wall curls. Only use a weight you can comfortably control your ROM with.