I don't think it's infrequent for spikes to grow some more! I don't know why. I have 2 phals that sometimes does it and sometimes not. I even have one phal which is very strange. It bloomed, dropped the blooms, grew more buds (4), now those blooms are getting old, and the spike is growing some more, with at least 4 more buds to come!
So, this one has been in bloom 6 months so far. My longest blooming phal was nearly 8 months!
I think that the spike growing again might depend on the genetics of the plant (since not all my phals do it), and I suspect there may be some environmental or culture related trigger.(since I don't see it every blooming)