The MonetDB server may become a victim of Linux kernel functionality called Out Of Memory Killer (or OOM Killer), responsible for dealing with excessive memory requirements. If the system reaches a point where it may run out of memory, OOM Killer looks for victim process and ends its life the hard way.
In most cases, a simple restart of the server will suffice, but when the killer enters during a complex update transaction, then the database may end up in an inconsistent state.