The Free On-line Dictionary of Computing (19 January 2023):

casters-up mode

   [IBM, probably from slang belly up] Yet another synonym for
   "broken" or "down".  Usually connotes a major failure.  A
   system (hardware or software) which is "down" may be already
   being restarted before the failure is noticed, whereas one
   which is "casters up" is usually a good excuse to take the
   rest of the day off (as long as you're not responsible for
   fixing it).

   [Jargon File]


The Jargon File (version 4.4.7, 29 Dec 2003):

casters-up mode
 n.

    [IBM, prob. fr. slang belly up] Yet another synonym for ‘broken’ or ‘down’.
    Usually connotes a major failure. A system (hardware or software) which is
    down may be already being restarted before the failure is noticed, whereas
    one which is casters up is usually a good excuse to take the rest of the
    day off (as long as you're not responsible for fixing it).