There have been certain instances in my life where I wasn't 100% comfortable doing something, but assumed I could pull it off if I practiced or prepared myself enough, only to freak out when the moment actually arrived. Like giving a speech practiced in front of a mirror or crowd of friends numerous times, or performing CPR on a real person even after being trained and certified. Just too many nerves, lack of confidence, and fear of making a mistake.
I know it's not exactly the same situation, but your wife's reaction could be simply for similar reasons, and not some deep underlying cause. It could be many reasons really, but I don't think it's anything serious to worry about.
I will say, I am very comfortable in my submissive role and have no desire to 'switch' with my Dom. If he wanted to try it, just out of curiosity, I would want to be able to give him that. And I would try to make it a good experience BUT I'm pretty sure I wouldn't enjoy it for myself or be very good at it. Some of the worries in the back of my mind might be: What if I get him all tied up then don't know what to do? What if I hurt him? What if I look stupid? What if he doesn't like it? WHAT IF HE DOES? What if this changes the dynamic between us? The list could go on and on. Endless worries that could cause the reaction she had. It might not seem like a big thing to a lot of people, but sometimes the smallest changes can be scary. Feeling out of your depth can be scary.
If you're perfectly content without trying out the submissive side, then the easiest thing would be to just forget about it. If you'd still like to try it, my advice would be to work it into a 'play session'. As her Dom, let her know you'd like her to help you experience a tiny bit of bondage. Maybe binding your hands behind your back so you're restrained while she is on her knees giving you a blowjob. Nothing complicated or too long. (Kinda a trust building exercise for both of you) Make sure she knows YOU are still the one in charge, she is just following orders and you will be there to help guide her. That should alleviate some of her anxiety and open the door for other possibilities in the future...slowly as she gains confidence and feels more comfortable. Could lead to a little self discovery and personal growth for you both OR stick to what works and enjoy what you've got.