No clue. I had none of the 'risk factors.' Never smoked, had a healthy diet, have been active my entire adult life, never been overweight, didn't drink to excess, no family history, yet Dx at 41. Go figure. Stress may have played a role, because I tend to be an anxious person (working on correcting that), and for a while had a job that would literally cause me to have heart palpitations and stomach pains. Left that job in late 2011. My doc said my tumor had likely been growing for 8-10 years, which would've meant it began around 2005 - 2007, prior to my stressful job.
Long story short: no clue. All I can say is that now I don't worry about every little ingredient in food because that didn't seem to make a difference. I also try to not stress about stuff and I currently have a job I love. I'm also trying to be more positive about things.
Unfortunately, I think most of us will never know what caused it. It sucks, but as others have said, a lot of it comes down to crummy luck. I know there's an actual, scientific reason this has happened, but will we find out what that is in our lifetimes? Who knows. Research is always ongoing, so maybe.
42 yr. old female
Rectal cancer Stage 3C T3 N1 M0 - Sept 2015
28 rounds radiation w/Xeloda - Nov - Dec 2015
2/17/16 - Surgery to remove rectal tumor, lymph nodes (2/20+), ovaries & fallopian tubes, temp. ileostomy
3/28/16 - 9/26/16 -12 rounds FOLFOX w/full oxi
Ileo reversal 10/27/16; Port removed 12/1/16
Lung mets confirmed 2/6/17
March-May 2017 - brain mets; brain rad. 5/9-5/29/17