Why do ICs need a specific power down sequence?
Because you could have a higher power rail bleeding out into a space that is only designed for lower voltages and burn out transistors. It's also good to avoid short term overcurrent on power down.


  Power supply sequencing can prevent damage and extend the life of your
  power supply and any sensitive DSPs and FPGAs in your design. In
  addition to controlling the startup timing to control current draw,
  the power supply rails need to stay within the voltage tolerance
  requirements of FPGAs. These tolerances have become tighter with new
  process technologies, to the point where 3% total core voltage
  tolerance may be necessary. Sequencing the power supplies in a system
  can be accomplished in several ways
  Source: https://www.powerelectronicsnews.com/technology/power-supply-sequencing-for-fpgas