HHVM Fallback on php 5.6
well as wordpress recommend PHP 7 now so most dev update there plugin or create it for php 7 or higher so when he uses hhvm which fallback on php5.6 it show some issue
monit can help but it can not solve plugin realted issue
It could be memory realted how many plugin you install i can see 549 in the screenshot
You should not use that much plugin