One theory as to why teenagers abbreviate everything:
Maybe cutting words off at these sounds is a little more common because the results are simply funny. Not that many English words end in ? or d?, and many of those that do have a bit of a silly ring: cadge, smidge, smudge, drudge, hodge-podge, fridge and such sit alongside a few respectable words like bridge and ridge and dredge. Even fewer words end in ? than in d?, and tend to be French borrowings or produced by the -age suffix borrowed from Old French: mirage, dressage, frottage, arbitrage… So my guess is that it's fun to end a word in ? or d? just because English doesn't do so very often.
Stan Carey cuts teenagers some semantic slack:
[Y]outh is a time for rebellion from, and reinvention of, the world being inherited, and this is as true of linguistic expression as it is of any other behavioural domain. Slang, as Eric Partridge wrote, is the quintessence of colloquial speech, "determined by convenience and fancy". It lets people experiment with language at their ease and pleasure, playing with it as they would play with paint or putty, sharing new shapes as though it were Lego. You don’t have to be a creative writer to be creative with language.