A 2L starter in five gallons is not underpitching by any stretch of the imagination. That's a roughly a 10-to-1 step. My bet is on wort composition. Banana is the ester known as isoamyl acetate, which is formed via a condensation reaction between isoamyl alcohol (a.k.a. isopentanol) and acetic acid.
C5H12OH + CH3COOH → C7H14O2 + H2O
The reaction shown above reads isoamyl alcohol plus acetic acid produces isoamyl acetate plus water. What this reaction means in layman's terms is that your fermentation generated isoamyl alcohol, and there was acetic acid available with which to form isoamyl acetate. Acidic acid comes from acetyl-CoA. We know that higher alcohol (isoamyl alcohol is a higher alcohol) production increases with biomass growth; however, so does acetyl-CoA utilization. On the other hand, overpitching results in elevated acetyl-CoA and lower higher alcohol production. This batch was neither overpitched nor underpitched, which means that something else went wrong. What was the original gravity? What was the wort composition? How much trub did you carry over from the kettle?