Provocative post, jrb. But, I think the answer is simpler: survival.

In the fish tank scene, Tess complains bitterly that Barzini is invading his territory, and "pretty soon I won't have a place to hang my hat." Vito and Michael tell him to be patient. Tess concluded that both were going soft, and that, when Vito died, Barzini would soon overrun the Corleone Family, in the process whacking Michael and probably himself and Clem.

Also recall in the same scene, Clemenza said, "Godfather, you said the time would come when me and Tessio could form our own family." Emphasis on the singular--"family." There never was, and never will be, a co-Donship. So, best case, Tess believed he'd have to either split up the family with Clem or fight him to the death. Either would have weakened him and made him more vulnerable to Barzini.

So, he decided to be proactive--sell out to Barzini. It'd buy him time to figure out what to do next. Standing still was, in Tess's mind, sure death.