So the phosphatase, chitinase (and the others) make the P and chitin that's present in the soil (from things like bone meal, guano, and dead bugs?) plant available (magically changing phosphorous into phosphate, which is used in every cell in/on the planet/plant in the ATP cycle…it’s almost like I remember biology…sometimes it takes an inordinate amount of time for things to percolate around in my brain). No idea why the worm pile goes crazy.
It sounds to me as though we're making sure that the plant has "full access" to the sources of nutrient in the soil by adding enzymes that start the process of breaking these nutrient resources (substrates) down into ions or at least into simpler things that bacteria and fungi can work on. Put alternatively, by having a boatload of enzymes in the soil, we're helping to ensure that access to nutrients will not be a limiting factor for our weed plants (all things being equal). And now I get your last point. Roots exude these enzymes (for whatever reason...bait for bacteria?), and in the process of malting, the grain brain gets serious about making roots, so enzyme development goes into overdrive and using malted barley in tea steals these freely available enzymes and puts a goodly portion into the rhizosphere (or the plant surface even), right where it's needed? Yeah, I dreamed that entire last bit up I think...but it could be how it works.
Still no idea why the worm pile goes nuts...I wouldn't think worms would be terribly dependent on the enzymes since they come with guts and mucus...maybe the barley enzymes get leaf mold going faster...food source easier to deal with for the worms so they have more time for sex? Yeah, I don't know about that...but it could be how it works.
This is why I envy soil science students (I was planning to be one, back in the day); he or she could just jot that question down for his or her next class (not about the worms, about the enzymes)...or maybe read Tim Wilson's, Microbe Organics page...or read the "Teaming with..." books (I've read them and think they're great, but I have a ways to go before I actually get it...it's complicated, and I’ve already admitted that I’m slow). cheers