You've got it backwards.
What happens is that the orgasm occurs and then the heart rate drops.
If you're suffering bradycardia from an event of some kind, then you aren't likely to become aroused, much less have an orgasm. Can't say it's impossible, particularly with some of the odd reactions meds can cause, but it's highly unlikely with any of the typical causes of a slow heart rate that one would be said to "suffer".
Mind you, a non emergent low heart rate isn't a barrier to orgasm, but I don't think you were asking about normative ranges, or when the pulse is just low from a non important cause (say, post meditation).
Typically, a bradycardic orgasm is referring to what happens in the body post orgasm, though the name obviously implies otherwise. It can start during orgasm, since it's not exactly prohibitive, but that's less common that it starting after.
Orgasm causes multiple cascades of body and brain reactions. One possible outcome of that is a deep relaxation that's actually akin to a meditative state, or like some stages of sleep.
That post orgasm relaxation usually doesn't drop heart rate low enough to be called brachycardia, but the basic mechanism is the same. Hell, it's not even that rare compared to stuff like sneezing fits, or difficult to control emotional outbursts. All kinds of weird shit can happen post nut. Nose bleeds, hallucinations, temporary paralysis etc.
Bradycardic orgasm isn't dangerous by itself. If accompanied by other symptoms, it can be. The two big ones would be dizziness/disorientation and chest pains. The chest pain might be a sign of other cardiac issues, and would need to be checked out immediately until and unless a cardiologist gives the all clear to ignore it.
Dizziness, by itself is only a concern if it doesn't fade quickly, or it comes with enough disorientation to make the person prone to trying to get up and move around before things settle back to norm.
If it causes fainting, then that's something that needs an immediate trip to a medical facility to be safe.