It seems client is sending DHCP DECLINE message which will not have any source address, same as DHCP discover/request messages. this could be the reason you are seeing 0.0.0.0 there.
If you know any specific machine with this behaviour then please if it's happening continuously or after any DHCP release time out, and a packet capture on the on the machine during the issue state can give more insight to sole the problem.
Thanks,
Vikas