This is directly from the ALPA By-Laws and Constitution, which is available on ALPA's website.

SECTION 7 - TERMINATION OF MEMBERSHIP
A. Membership shall be terminated by expulsion when a member remains in bad standing for a period of two (2) consecutive months because of nonpayment of his dues, assessments, or fines, as provided in Article IX, or when he is expelled as provided in Article VIII, or when terminated as
provided in this Article; or by voluntary resignation submitted in writing by the member (recommended via certified mail/return receipt) to the Vice President-Administration/Secretary of the Association.