Help with issue at Nursery where I work

[Mrs Frog]

I am leaving my employment to work at another nursery,a smaller one,at My leaving interview I was told that a parent had made a complaint about me 3 weeks ago,this is the first I have heard of the complaint,I raised my voice to a child who had pushed 2 other children from the top of a slide whilst they were on the ladder,I was some distance away so I had to raise my voice which I never do,why are they only just telling me now after I have handed my notice in?Some advice please thank you