Someone asked him a question about him pushing gun control and then he went on a 5 minute long tirade about infrastructure and children's drinking fountains in schools (nothing about guns).
Looks like he screwed up the coordination of reading the wrong pre-written response to the wrong question.
Fucking sad.
My take: remember how Circle Gremlin talked about accepting questions in advance? They prepared a question and submitted it in advance, but the RADICAL ISLAMIC TERROR ATTACK happened after the submission deadline. The reporter ad-libbed and BIden did his rehearsed answer.