checking dmesg, there are OOM errors:

[ 1061.946668] oom-kill:constraint=CONSTRAINT_MEMCG,nodemask=(null),cpuset=cri-containerd-c3aa5aaa98e869f97a404b8a1bc27cbdfbdcd85960dcb021930bceffe70c6061.scope,mems_allowed=0,oom_memcg=/kubepods.slice/kubepods-burstable.slice/kubepods-burstable-pod568dc473_4171_4b3f_9e8f_61ad43186fa1.slice/cri-containerd-c3aa5aaa98e869f97a404b8a1bc27cbdfbdcd85960dcb021930bceffe70c6061.scope,task_memcg=/kubepods.slice/kubepods-burstable.slice/kubepods-burstable-pod568dc473_4171_4b3f_9e8f_61ad43186fa1.slice/cri-containerd-c3aa5aaa98e869f97a404b8a1bc27cbdfbdcd85960dcb021930bceffe70c6061.scope,task=ruby,pid=11345,uid=100
[ 1061.946678] Memory cgroup out of memory: Killed process 11345 (ruby) total-vm:955300kB, anon-rss:771480kB, file-rss:6780kB, shmem-rss:0kB