Why is my Transaction Log Growing in My Availability Group?

Dear SQL DBA

A database transaction log is expanding, even though the DBA is running log backups and doesn't see an open transaction? What's going on with this Availability Group?

Timestamps in this episode if you want to skip ahead:

00:49 What's new @ SQL Workbooks 03:22 Info on SQLPASS Summit Content Survey & a heads-up for aspiring precon speakers 05:35 This week's question about t-log growth