Hydro disc brake...
calipers and master cylinders have little nooks and crannies that can hold small air bubbles at times. It's the same way with automotive brakes but they never get turned upside down normally. What likey happened was there was a small air bubble left in the MC when you disconnected the bleed kit. When you stored the bike upsidedown it followed the tendency of an air bubble in liquid and rose to the highest point in the container, that being the caliper. A small air bubble in the MC at the top of the fluid resiviour won't hurt anything unless it gets pushed into the line. The bottom line is, storing the bike upsidedown likely caused a bit of air to migrate. A rebleed should take care of it. Also most bicycle hydro brake systems are "open" systems. That is there is an intentional air pocket in the master cylinder, usually above the fluid resiviour and seperated from it by a rubber seal. This is to allow for expansion of the fluid as it heats up and prevent "lever pump", i.e. the lever gets stiffer up to the point that the brakes are constantly applied. The brakes/fluid mus then be allowed to cool down. Anyway an open system prevents this. But if that seal isn't quite perfect, when turned upside down it can allow some air to get past and it ends up in the system where it doesn't belong, in the lines etc.
So just rebleed them. If that doesn't take care of it then you likely will need a rebuild on the MC.