GND Bounce
Large change of current on Vdd/Gnd pins (inrush current) due to multiple outputs changing simultaneously causes induced voltage on GND plane:
Larger the inductance, larger the change in current, larger the induced voltage.
Two ways to reduce Voltage: Reduce Inductance : More vdd/gnd pins (inductance in parallel reduces total inductance), better packaging (different packages have more/less inductance than others). Flex 10K20 240 pin package has 19 Vdd pins, 18 Gnd pins).
Reduce di/dt : slower slew rate!!!!