I write embedded apps. I've been on teams that use IP in DVRs and more so it's up to us to decide what actions to take when the DHCP renews. It appears the maker's coders have decided to do a drop on DHCP renew. It's perfectly understandable from my perspective. However as I find more of this work going overseas it's often impossible for us to get them to change the code.
So unless you find the code that is running these devices the real why must be kept under wraps.
Hope this clears that up.