Mass channeling a passage wider than 2 tiles is likely the issue, especially if there is no "walls" underneath. (EG, its been mined, THEN channeled)
This is because dwarves obey a rather silly (but I cant really think of a better one) algorithm that prioritizes "last defined tile" with a prerequisite of "reachable", and "not currently tasked by another dwarf". The same algo is used for constructions and whatnot.
The problem happens when you have more than one miner, and you mass designate an area to be channeled. Rather than be sensible about this, and not channel tiles that other dwarves will need to stand on to complete their tasks-- dwarves will slavisly channel out the nearest, pathable, and not-already-active designated slot to mine. (there's a little more to it than this, but this is the basic algo) This means that if Urist McMolasses-Ass picks the tile in the "middle" of the 3 tile wide channel, and SpeedyMcUrist gets there at the right time, Molasses butt dwarf will be standing on the tile that is being supported by speedy's tile, speedy will channel out his tile, which then removes the support for the tile Molasses butt was standing on, and there is a cave in with molasses butt at ground zero. HOORAY. Dwarf falls, has stuff land on him, he gets all kinds of buggered up.
To alleviate this problem, I do things this way:
1) no more than 2 miners. No, For real. This is necessary unless you want to seriously micromanage.
2) Designate channels no wider than 2 tiles. After the intial channel is cut, do it in strips 1 tile on each side, as needed, to widen the channel.
3) Pay attention to support!
4) Try not to "Mine, then channel"-- it causes many problems.