Short explanation: Dumbass move but won via sheer dumb luck regardless.
Long: Japanese Mahjong disallows winning off other people's discards if you have discarded one of your winning tiles before (this is called being in a state of furiten). Declaring riichi means announcing you're one tile away from winning to the other players, in exchange for getting another han (hand scoring 'point'). You're also locked into your current hand and can't change waits (or hand composition, equivalently).
Declaring riichi when you're in furiten is a dumb move because you're now permanently locked in furiten and can only win by self draw (i.e. tsumo). But Peko lucked out anyway and immediately draws her winning tile one turn immediately after her riichi declaration (i.e. ippatsu), which also grants her an additional hand 'point'.
