Not hard, dude. You should know that the people-who-search-for-x-also-search-for-y is inherently built into WHEN-people-search-x-other-people-are-also-likely-to-search-y, since people who are searching on "accident" and not finding what they want aren't likely to wait 4 weeks before trying "fatal accident" - instead they are going to immediately try it, meaning that WHEN that person searches x it is also likely he will search for y.

The other (non-mutually exclusive) explanation here is that different people are searching for the same thing (e.g. a news item they saw on TV about a recent car accident) but using different terms to do it (e.g. accident, car accident, fatal accident).