Perhaps you'll need to work around the problem by asking an additional question in advance of the mandatory one to establish whether the 'other' option will be required. Your trouble stems, I think, from the fact that you're really asking for two pieces of information - what reasons there are, and how much those reasons influence the behaviour. You might be able to make it work by dealing separately with them.
Also look at someone's
solution for the opposite in case it suggests a solution to you.
Regards.