Quite a common citrix forum one this and can have many causes/fixes.
One that is quite common is MS patching, whilst updating one of the lab images i was working on did notice with a few XenDesktop machines (windows 8 the OS btw but can cover any variant of windows) that the write cache growth was quite stark soon after reboot. In my haste had not let the MS patching cycle complete 2 reboots – after letting windows update do its thing while i had my pvs imge in private mode it was looking to do a 2nd reboot to complete. By sealing up after the 1st reboot and setting to standard mode it just went and started putting the patches on my VDI’s whilst in standard image mode though causing the growth. Simple enough to fix by setting back to private image mode (or of course maintenance mode probably better) and allowing the patching to complete 2 reboots before reseal.
Shout out to http://xendesktopguy.com/ for the fix