Yes, when you let your water get messed up. High NH3
/NH4 or nitrates will make them loose thier crown....the dont really drop it, it kinda disentigrates for a week or so first, then they pull into thier tube for about 2-3 weeks, and if you fix your water, it will make a COMPLETE comeback!
I know from experience (aka stupidity) that you must keep you water good for dusters.
Here is mine, after loosing its crown for three weeks, it has now made a total comeback.
Whatever you do dont throw it away for a while!