That is from the combination of the power as it makes landfall and goes inland + then the stall and magnitude/scope of the extreme deluge + the total duration including what might happen in the longer range. The atmosphere has written a scary meteorological script.
Harvey appears to have undergone an eyewall replacement in which an outer, larger-diameter eyewall forms and flourishes. That slowed the rate of increase in maximum wind velocities, but the size of the windy core expanded, which is not a good thing, as it widens the core wind/surge zone.
Also, the central pressure has continued a downward trend — storms which are intensifying as they approach the coast tend to have extra vigor — and is now dropping through the 940s (millibars), which is representative of a very powerful hurricane.
Landfall impacts: Intense surge/waves with severe coastal flooding and wind strong enough to produce structural damage and not only power outages but widespread, extended ones. Also as is typical with landfalling hurricanes, potential for very localized tornadoes.
Then: Slow movement + torrential tropical downpours = a deluge which is expected to be exceptional even by tropical cyclone standards, possibly record-breaking. Flooding on time/size scales from flash/local to long-lasting/widespread.
Meanwhile: Onshore wind flow will continue, from near the center of circulation well up the coast to the northeast, where water along that stretch is already above normal astronomical levels. That could play a role in the final chapter, and in the meantime will cause beach erosion and an elevated rip current risk, and also back up rainwater trying to drain out of the watersheds.
The final chapter(s): In the longer range scenarios ranging from staying over land and bringing even more heavy rain to flooded areas, to finally accelerating out to the northeast but doing so over the water and restrengthening and making another landfall. More uncertainty at that time frame but not much optimism then either.
Stay tuned to The Weather Channel for the latest, and stay safe!