If you want to stay present and embodied during an imagery experience, you can listen while standing up, leaning against a wall; or sitting with eyes half-open; or while walking, even.
Or even seated with your eyes closed, if you notice yourself starting to float, you can also focus your attention on the feel of the support of the floor beneath your feet; the chair beneath your bottom or against your back; or the feel of your breath in your belly. By putting your awareness on these body sensations, you automatically drive consciousness back into your body.
Either way, it's no cause for alarm (unless you're driving, which, as you know, you're not supposed to do while listening to guided imagery. And this is why!)