NicholasW NearsWeltanschauung Fall here means simply pass into a state. This sense might have been a gradual change from a different sense (I can't get to the OED to check), which is why I doubt whether it's a metaphor.

the OED has XI (11) general senses for to fall, of which I refer you to number VII. to pass suddenly, accidentally, or in the course of events, into a certain condition.
38. a. Of persons: To pass (usually, with suddenness) in, into, to some specified condition, bodily or mental, or some external condition or relation. e.g., fall into unhope, fall to sleep, "He was fallyn in prosperite" (Chaucer), fall to pieces, fell out of favor, fell into a rage
b. to fall in love: to become enamoured. Const. with. Also transf. to become very fond of, or devoted to.
"Would'st thou then counsaile me to fall in loue?" (WS)

but I digress. returning to the evolution of senses, we traverse from
I. to descend freely (opposed to rise)
II. to sink to a lower level (ditto)
III. to lose (suddenly) the erect position (opposed to stand)
IV. to move precipitately or with violence; to rush
V. to be determined to a specified position; to have a certain incidence
VI. to come casually, or without design or effort, into a certain position
VII. to pass suddenly, accidentally, or in the course of events, into a certain condition

so for VI we find senses such as:
35. Of persons. a. To come by chance into a certain position. Now chiefly in phrase (of biblical origin), to fall among (thieves, etc.).

OED:QED - no metaphor

http://home.mn.rr.com/wwftd/