When multiple output drivers switch simultaneously, they induce a voltage drop in the chip/package power distribution. The simultaneous switching momentarily raises the ground voltage within the device relative to the system ground. This shift in the ground potential to a non-zero value is known as simultaneous switching noise (SSN) or ground bounce.
The ground bounce voltage is related to the inductance present between the device ground and the system ground, and the amount of current sunk by each output. It is given by V = L × di/dt