Because the buffer is full, 30 minute limit? It will do the same thing on HD channels. If you are saying it starts playing when the buffer is less than 30 minutes, no idea why.
Home Theater - OTA TiVoHD/1.16TB/NSZ-GS8 GTV
MBR - OTA TiVoHD/660GB/NSZ-GT1 GTV
Den - OTA TiVo Premiere/2TB/NSZ-GT1 GTV