Years ago, my aluminum handlebars on a single bike sheared off right beside the stem just as I mounted it. The sudden loading as I put my weight on them was the last straw I guess. And I'm only 140 lb., never raced. The bike did have a minor crash several years before when I lost control running onto a gravel shoulder and dumped. I suppose the handlebars took some impact, but I never really thought about them after that. Also saw a bar break in the same place when the rider mounted and settled onto the bike, same mechanism. Crash history unknown.

Check your handlebars for sag. If the centre-to-centre distance at the drops is not the same as when you bought them, they have bent somewhere. We know how much aluminum likes bending, because a bend is really a crack. And replace them when you crash.