I have heard the same about too much GFO.I have been told that sometimes corals have a bad reaction to it. Don't want to put words in his mouth but I think Jeremy told me that he attributed it to phos guard pulling out phosphates too quickly and could result in too much of a change too quickly.
As usual - anything sudden is bad.
Although since it can affect Alk levels as well, could be that, not phosphates directly.