serpent star loving arms so easily any body knows why?

today while moving things to a new tank, i tried to move one of the serpent star fish, a black one, they r 2, one of them lost two arms while moving, every time i try to hold it from an arm it broke soooo easily, so i catched it from the disc.

anybody has any idea why its arm r so easy to be broken? the other one isnt, and all the 3 serpent in my old tank which i have from years never faced this problem with them.