Characters
I couldn’t help but overhear, probably because I was eavesdropping.
— Richard ‧ Ally McBeal
Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker admits to overhearing a conversation, then undercuts the excuse by conceding it happened because of deliberate eavesdropping. The joke is the self-cancelling logic: "couldn't help" becomes "chose to." It is a cheerfully shameless confession of nosiness, delivered as if it were an explanation.
Context reviewed