EW is used for attaching certain adverbs to question words:
+--EW-+ | | about when did it happen ?The EW link is used because none of the other adverbial modifier links (EA, EE, EN, EZ, etc.) are appropriate for this situation, and would allow too broad a range of possible linkages. Although the QI link could fit in this situation, it is meant for forming indirect questions ("I wonder when it happened?"), and is not appropriate for such direct questions.
Grammar Documentation Page.