What would cause someone to fall asleep so easily-- like texting, sitting in a waiting room, reading, talking?

Many causes. You are describing excessive sleepiness, a symptoms with many causes: not getting enough time in bed, sleep apnea, narcolepsy, idiopathic hypersomnia, depression, anything that significantly impairs sleep (restless legs syndrome, periodic limb movement disorder, insomnia for any reason), alcohol or drugs, withdrawal from stimulant drugs or medicines, etc. See a sleep medicine specialist.
Deficit/narcolepsy. Your system has some automatic functions that are designed to promote long term health thru adequate sleep. If u do not get proper sleep for your age/activities over the long haul, your system may shut down easily to catch up.Ineffective sleep (sleep apnea) can cause this or a condition called narcolepsy. A simple discussion with your dr should refine further info needed to pin it down.