DHCP or BOOTP does not work on diskless machines with NFS-mounted / partitions. These machines need fixed IP addresses, but it still makes sense to use DHCP or BOOTP to optain other IP parameters (nameserver, routers, etc.). Pump seems to unconfigure the interface before obtaining an IP address for it, and this makes the / filesystem unaccessible crashing the machine.
While a patch is welcome to fix this (is should be configurable on a per-interface level via /etc/pump.conf), I'm not going to add it myself.