There's growing evidence, especially in the past few years with better studies, that suggests HPV is a significant driver, if not the most significant driver, of the increase in colorectal cancer among younger adults. I suspect it's been a disfavored explanation because of certain implications--implications which should be irrelevant and not even necessarily true, but I digress. The HPV vaccine should in theory be protective[1] so in the next decade or so it might become more clear even in the absence of additional direct investigation. Likewise, we should expect the incidence of oropharyngeal cancers to decrease, which probably not coincidentally has also risen among younger--20-50yo--adults. Notably, the HPV link is more clearly established.
[1] HPV16 and HPV18 being the variants most often identified in HPV-associated colorectal cancers[2], and which are targeted by HPV vaccines as they're the variants primarily responsible for cervical and anal cancers.
And the last decades we’ve had a new unknown cause of colon cancer increase in young adults.
My money is on plastics, but will be hard to prove.