dropping spines could be from several things , example water parameters can be off / stress / sick and dying / someone picking at him / parasites .
now if you find the skeleton turning white where the spines are missing it is most likely slowly dying , or telling you your parameters are off , or it may even be parasites,
inverts are always the first affected by drastic water quality changes,
I always experienced if it starts dropping spines rapidly they very seldom recover ,
I actually experienced one of my urchins shoot all it's spines at once , he lasted about a 1/2 hour before he passed , he was almost 5 years old I got him as a hitchhiker on some live rock , he was the size of a thumb tack head .
Fish are like lifesavers
They come in different colors