If their static IP is configured in a specific VLAN and their uplink VLAN configuration does not match that VLAN it will take a DHCP address from the VLAN it is receiving.
For example: your static IP is in VLAN 20 and you have set VLAN 20 as native in your uplink port, but the uplink switch/other appliance has a different VLAN as native, then you will receive DHCP from that VLAN instead.