Is there a reason why an app would want all of my contacts?
Definitly not! That's nothing like what a developer needs. And a developer requiring you to send that I'd avoid in the future as well. All that might be needed can be provided by e.g. adb bugreport (which is the most comprehensive data collection known to me to be useful for a developer concerning his development). Requesting all your addresses is harvesting, and rather used for spamming.

TL;DR: I would NOT send that information. You might contact that dev and ask for an explanation, and then maybe decide different. But I cannot believe this being a "mistake" or simply "carelessness".