War Plan Red called for the complete take-over of all British possessions within the western hemisphere, the invasion of Canada was the first priority and after that then the islands and other colonies would be invaded.
Without changing too much ( and I know what I'm about to say is unlikely to actually start a war) I can think of two "possible" scenarios.
1.) Have the London Naval conference turn out much worse then in OTL, making the U.S and GB build up their navies in response to this. More preparation would go to Plan Red (which already was being done until the mid 30's) and some leak occurs regarding plan red and war happens that way.
2.) Have the U.S much more demanding in the re-payment of their debts, GB is having more economic issues and isn't able to keep up the payments etc. From their a larger anti-British sentiment would arise, and say sometime in the early to mid 1930's have someone like Charles Lindbergh assassinated/killed during a patrol in the Puget Sound or maybe even a "Canadian raid" over national lines.
You would also need to have a different Premier and President that actually wanted war and didn't understand that a strong Britain is a strong U.S and vis-versa. I can't offer suggestions as to who those people might be however.