NAT - Transmission scans own ports for no apparent reason

[deadpenguin]

Whenever a torrent is active, Transmission would scan the ports of its own IP address, usually starting with 60000 and climbing, but I've seen it scan from 30000 onward from time to time.
The speeds are fine, the port forwarding is fine, there are no crashes and the bug is reproducible with the default settings. Usual number of peers is 30-40, the router can handle several hundreds.

Relevant info:
OS X 10.6.3 (10D537)
3Com ADSL wireless router 3CRWDR101A-75 with firmware 1.12.01.A
Transmission version: 1.91 (10268)
"Randomize port on launch","Automatically map port", proxy - OFF
PEX, DHT, encryption, blocklist - ON

The issue persisted with all recent versions of Transmission (back to 1.7-ish), all recent versions of OS X (back to 1.5) and all recent versions of the router firmware (back to 1.0).

Haven't tested 1.92 yet.

[deadpenguin]

Tested 1.92 (10363), still port scans for no reason.