This is the technique I switched to and I'm fairly convinced that maintaining tension particularly in the early stages of the knot is important - it's basically a chinese fingertrap and you know how those work; the only way out is to introduce slack into the far end. I think that when the FG knot fails, it's basically the same thing happening: the mono end of the wraps isn't tight enough so the slack works its way back through the knot and opens it up.
Disclaimer: I am pretty good at knots but I'm not a knot scientist or anything.