I'm far from an expert, as you will see from my posts, but both of my phals did that when they were at the end of their blooming cycle........ or rather, when those particular blooms were at the end of their cycle. My big white phal (a NOID) still has loads of blooms (it's got three spikes with double cascades of flowers, but two curled up on that last week, and then fell off. The ones remaining are still okay, but starting to show signs of wear, and since I can't see any other reason, I'm just figuring it to be the aging process. There are two beautiful new leaves growing, and a very healthy looking root peeking out from under the leaves.
Maybe someone who knows more than me will have a better idea though, so let's see what they have to say. I'm learning, right alongside you.