15% is not very high as long as the Bush name has been around Republican politics. And Alabama shares some border with Florida, plus Alabamians flock to the Panhandle beaches. Lot of connections with Florida so many have just heard a lot about Bush and Rubio, especially in south Alabama.
I think they'll drop off once voters start paying closer attention and see a couple more debates and hear the Bush and Rubio stances on issues.
This is not the time to get an accurate poll in Alabama. People are not paying a whole lot of attention to politics. SEC football season is upon us. Priorities, people, priorities. /s