I think from what you say that as you were in a relaxed mode, possibly :
You went to move your foot and did so MORE than expected and you hit the clutch by mistake - hence no braking ....
- you missed all the peddles and hit the 'floor' - or possibly even far enough to hit the (left) foot rest.
As you found the brake and regained control we can likely assume that there was no error with the brake pedal temporary or otherwise.
So if not mechanical then personal.
The nearest I can relate this to is when the brain is 'thinking' something else and you sub-consciously go to 'do' a 'normal/automatic' action, that there is not enough 'thought' into processing properly and 'miss' the precision of control needed.
You immediately went on alert, and then regained control - when the brain was concentrating. During this period you can go through a process of 'lost' a kind of stabbing about to regain positions too.
So pleased that it all turned out all right. A real reminder that your space is for the unforeseen emergencies !