Static, Capacitance, cross talk, and noise on a pin or trace can cause the voltage on it to be higher or lower than a reference voltage of vcc or vss. If a pin is not used, but soldered down to a trace on a breakout board, it acts like an antenna, creating the floating state, because it can change voltages from one millisecond to the next due to anything it picks up. It is essentially the same issue as a non-debounced switch. While you don't have an interrupt on an unused pin, the processor still sees and tracks the changes.
A floating pin is to a microcontroller like a drippy sink is to a