Query_Events table GROUP_BY_PREPASS_FALLBACK event type

we have a simple query (see below) running on an 8 node vertica cluster (6.1.2). it throws the event (Group_by_prepass_fallback) on each node. what is this event? i can't find a description in the manuals or forums. in general, where are the events documented?  thanks.  / kevin

simple query: select a, sum(b) from foo  group by a  order by sum(b) desc;  (foo is a large table)