According to this article in Politicker, it was the Sandy Hook massacre that prompted Democrat Joe Wenzel to challenge Rodney Frelinghuysen in the 11th Congressional District.

According to Wenzel, CD11 incumbent Frelinghuysen wasted his seniority in Congress by failing to address issues of weapon control as relating to the safety of children.

This Politicker article also provides some background information about Wenzel’s career and qualifications.