What causes this in SGC/PSA holders?
 

Educate me so I can educate my son who is now getting into buying graded cards.

I no longer buy/trade raw cards from people/sellers I dont know (tired of being burned), I look more closely at the holders. I took these samples from ebay so if they are your cards I apologize in advance.

Should one be worried about these types of distortions and what causes them?

The rest of these holders look fine.