Reedit: The likely point of the introduction of physics to Ash was to make it impossible to guard oneself from being attacked in the underworld: Hellstone is obvious, plentiful, and well-guarded. For game balance, the well-guarded aspect is, as you might be able to surmise, pretty damned important (no pun intended).
Whenever a block is placed or removed next to an unsupported mud/ash block; it has an approximate 1 in 4 chance of taking note of this change and falling to the ground. Of particular interest is the fact that this slide can trigger any blocks next to the now falling block to fall in turn; and any blocks near the landing site as well.
Wether it falls or not is random; it is not determined by the number of chained unsupported blocks.
The best way I've found to mine Hellstone is to carry a Staff of Regrowth or better on my person at all times, as well as an Obsidian skull to negate the damage from the Hellstone itself, and always burrow in from underneath the top of the deposits, leaving the top block of every column.
The size and commonness of the deposits more than justifies the "waste" that is the top block of every column, as you can get to the next deposit faster and with less tricky maneuvering.
Edit: My point, basically, is that the physics change to Ash and Mud pretty much negates the old guide to digging out Hellstone, making it a whole lot more difficult.
Then again, Hell is supposed to suck (c.f. The Nether, Hidden Fun Stuff)