So, one common misconception is that "stretching" will decrease injury. In fact, studies have shown this is not the case. Stretching is also very different from warming up. Active warming up may decrease the incidence of pulling a muscle, but tendon rupture is a VERY different animal.
As far as tendon rupture, I wonder, were you on any medications (specifically a fluoroquinolone abx), taking any supplements/herbs, any prior issues with that arm (stress, broken bones, car accidents, other trauma)? Also if you consider anatomy, the bicep muscle does not limit range of motion of the arm, so in order to rupture it you would have had to been pulling actively against the force of your other arm muscles (assuming extension of the elbow), which is somewhat of an interesting thing to consider.