Incident: American B738 near Dayton on Apr 1st 2011, medical emergencies
Reading the report it appears there may have been a slow leak and not a R/D... almost indicative of no air to the packs.
Below is for supposition purposes only and nothing else.
Are "No Bleeds Takeoff" accomplished from DCA for performance enhancement?
If so, is it possible the APU was inadvertently switched off while performing the after start flows.
Despite briefing the "No Bleed Takeoff" procedure, I've seen eager F/Os still reach for the APU, switch (merely out of habit) many a time and hd to reminding them not 'UH UH UH... don't touch the switch.
DCA departures can be rather busy for crews with close attention being required after takeoff to avoid restricted airspace thus leading to distractions relating to attention to the jet itself.
It has happened in the past and it will happen again as reconfiguring the engine bleeds and isolation valve switch on the A/C Panel.