"Solid" fuels are not all completely solids.
Some are actually pastes — viscous liquids, often colloidal suspensions — these can (very slowly) ooze. Some are pastes until they are cured; if not properly cured, they can remain liquid.
Some are particulates with binder chemicals. If the binder ages, liquifies, or otherwise becomes less effective, the particles can fall out.
Some are themselves solids that can liquify at certain temperatures below ignition temperature; if heating happens, this can result in liquefaction.
Potassium Nitrate and Sugar fuels can be liquified by humidity and/or temperature, and is crystalline and thus fragile and frangible. Vibration can render it cracked or even powdered, and it's hygroscopic; in wet enough conditions, it dissolves in water, and can result in sticky flammable messes.
Zinc-Sulfur propellant and black powder propellant are pressed powders - vibration can cause particles to come loose and "leak". (corollary: Never store a black powder rocket next to your subwoofers...)
Ammonium-nitrate & Aluminum or Aluminium Perchlorate & aluminum fuels use a binder, typically Hydroxyl-terminated polybutadiene (HTPB); the binder can potentially degrade over time resulting in fuel particle losses.
Hydroxyl-terminated polybutadiene (HTPB) itself is used as a solid fuel itself in a hybrid design; it's oxidizer is Nitrous Oxide.
Gunpowder & wax is a rarely used colloidal solid propellant; thin layers of wax are used to bind together the powder and reduce the powder's reaction rate. The wax can melt at not too terribly high a temperature, and a hot summer day can result in such propellant slumping or even running. Parafin, while melting at 122°F or so, will slump at about 80°F, so if experimenting with wax and powder mixtures, use beeswax, and even then, don't leave it in the car on a hot day - beeswax melts around 145°F, and a closed car on a hot summer day can hit 160°F readily.
Any hybrid rocket can have leaks of the liquid component, while still having the solid.