<VV> Bent turbines - just wondering

[Grant Young]

I have now rebuilt close to 100 Corvair turbochargers, and am still perplexed when I get one like I did today (probably 1+ in 10) that has the large outer vanes of the turbine wheel bent over or broken off. As fragile as they are, I am surprised that they don't always break, but that might be a heat related event. I think this would indicate that something big is going through the exhaust system to the turbine intake. I am wondering if anyone has any experience on this and could explain what might be happening? They have all been 150 HP units, BTW.
Thanks,
Grant