SQL Server Page Life Expectancy sharp drop
How about Database pages, Free list stalls/sec, Page reads/sec, Page lookups/sec, Readahead pages/sec? Their behavior, correlated with the event, would be very useful to know. Similar, all the counters in Memory Manager category.

Could be a query that scans a large amount of cold data hits the server. As it reads the cold data, it evicts pretty much everything else in the BP, including its own just-read pages. After the event the BP slowly start building up the hot pages and PLE start to increase. Counters in Access Methods object could confirm this.