A number of verbs have something called a second aorist, which uses the imperfect (secondary) endings instead of the characteristic alpha. I don't know that it's entirely predictable when this is happens -- often I think it's when the aorist uses a stem which is shorter than the present (as some verbs in nu, for example).
IPHIGENIE: Kann uns zum Vaterland die Fremde werden?
ARKAS: Und dir ist fremd das Vaterland geworden.
IPHIGENIE: Das ist's, warum mein blutend Herz nicht heilt.
(Goethe, Iphigenie auf Tauris)