I would venture to say it's the nicotine...some people have more tolerance for it than others.
I have only been sickened by two cigars. One was a RP Edge that I smoked on an empty stomach...was cigar #2 in my smoking career. I missed the "eat before you smoke" memo.
The other was a private label at the Bull and Bear in Naperviile. Tchariya can attest that about half through, i started to turn a little green. Had to set it down for a bit, then smoked a little more.
Normally, I just get a little buzz from the stronger sticks. But no..