Broken Fan Blade On 2001 Chevy Blazer

While driving down the road a couple of the fins on my friends 2001 Chevy Blazer fan blade completely broke off and a couple more were cracked. One of the fins impaled the fan shroud. He had just replaced the fan and the radiator because the same thing had previously happened. Any ideas as to what could be causing this? He made sure that everything was tight before starting the vehicle. Thanks

The next thing to check is the thermal fan clutch.