I think what we're seeing here is a mix of several dev goals conspiring to bring that proc rate down:
- Try to free up some GCDs for covenant abilities, Echoing Shock/Elem Blast. (Shorter Flame Shock duration, on single-target costs us 1.25s per minute per target)
- Try to reign in burst from the higher LvB damage, + Echoing Shock, + Ascendance, +Windspeaker's Lava Resurgence
- Try to reduce our casting mobility (To offset new 'mobile value' stuff: Echoing Shock, Spiritwalker, LolStaticDischarge?, HST, Windspeaker's Lava Resurgence)
- Try to get that first talent row more balanced (Echo of the Elements is significantly less valuable with a lower proc rate, )
- Create room for extra ES GCDs produced by High Voltage conduit, as we return to having high mastery? (Since ES is lower prio than LvB, aside from capping)
- Shift Ele more to a cooldown-based spec than a resource-based spec (high surge proc rate effectively removes LvB cooldown)
I'm just guessing that those are their goals, but I think they're all pretty obvious, and it is a fairly effective solution to them all.
Going from 20% to 10% proc rate is a stark shift, but I'm not convinced it will ultimately feel bad. Obviously for some players it will, but it's not like Ele is regressing to "immobile turret", nor will we lack for hard-hitting burst lineups.