I'm not sure to tell you the truth. Over the last several months I have definitely been making some changes to the tank, reason being is a flood, which cascaded into many other issues. Since that time I have been battling a red turf algae outbreak. I have changed salt, lights, lighting period. But what I think actually triggered the spawn was a slight rise in temp and a lighting cycle change, halides are on for a shorter period and the actinics are on for about an hour longer.
That's all I can think of that may have caused the spawn.