Training tragedy at Belmont claims Caixa Eletronica

[casp0555]

I was talking to an exercise rider at Sam Houston about this Saturday and he said a similar accident occurred that morning. He said he thought both riders were okay but one of the horses was injured. Again a horse lost a rider and ran into the other.
Understand that I am not very knowledgeable about training/working horses, but is this type of accident common?? Seems very dangerous for both animal and human.