On further reflection I think I need to revise my estimate down somewhat. Thinking on it further my 30% estimate is conditional on general trends that I think are more likely than not to occur but I did not correctly incorporate them into the estimate for secession. I think 10-15% is probably a better estimate taking that into account.
I think the political pressure for secession will stem from an extended period of economic weakness in the US and widespread fiscal crises in states like California and New York. If, as seems likely, federal aid is seen to go disproportionately to certain states that have the most troubled finances then the states that feel they are losing out will begin to see secession as an attractive option. My original estimate did not sufficiently account for the possibility that I am wrong about the economic troubles ahead however.
I would still be willing to take a bet at these odds, given some reasonably clear-cut definition of "attempt to secede".
I would like to propose this as a thread for people to write in their predictions for the next year and the next decade, when practical with probabilities attached. I'll probably make some in the comments.