Sean is correct. The cracks you describe are typically caused by prolonged exposure to heat. I have a pair of beater e Wires that I have stored in my car for several years. The lenses are filled with tiny stress cracks, and they are ok unless the sun is directly in my line of sight, which causes the light to pick up every little crack.
I would not expect over-tightening of the frame to cause this, as the Torx recess or screw threads would probably strip before any lense damage could occur.
In rarer cases, the lenses may have been defective from the factory, perhaps due to thermal shock when the lens blanks were made. The first pair of Ruby Penny I received brand new from an eBay seller had one lens that was terribly stress cracked. I arranged an exchange directly with him, as Oakley will not honor warranty of items purchased through eBay or other non-authorized methods.