Some switches use the Spanning-Tree Protocol (STP) to negotiate a loop-free path through a root bridge. While scanning, it can cause each port to wait up to 50 seconds before sending data. This delay, in turn, can cause problems with some applications/protocols such as PXE, DHCP and DNS, of which MAAS makes extensive use.
To alleviate this problem, you should enable Portfast for Cisco switches (or its equivalent on other vendor equipment), which allows the ports to come up almost immediately.