Not sure if anyone has considered this:
Alot of times, horse manure is mixed will old wood shavings. Those shavings can take a heck of a long time to decompose, and actually suck nitrogen from the soil in the process - I'm not sure if it would cause a worm die-off, but it sure keeps the plants from growing. I learned this one the hard way, sheet mulched a whole front yard a few years back, and covered it with a foot of manure/shavings that had been sitting for over a year....had no idea why things just did not grow. woops.