With NetScaler 11.0 build 71.18, High Availability issues may rise: the HA NetScaler reboots after an upgrade to 11.0-71.18 and generates crash files while performing a force failover. Also it may cause several failover in the HA environment. Citrix fixed this issue with NSOS 11.0 build 72.xx and newer – NSOS 11.1 and 12.0 contain the fix, too.
For details see https://support.citrix.com/article/CTX230126.