I found the slide in the curriculum that got me confused.
"There is one VTP server switch, S1, and two VTP client switches, S2 and S3.
S1 / \\ S2 S3 \\ \\ S4 (new switch)
S4, which has been previously configured as a VTP client, is added to the network. The revision number of the switch S4 is 35, which is higher than the revision number of 17 in the existing network. S4 comes preconfigured with two VLANs, 30 and 40, that are not configured in the existing network. The existing network has VLANs 10 and 20.
When switch S4 is connected to switch S3, VTP summary advertisements announce the arrival of a VTP-enabled switch with the highest revision number in the network. The animation shows how switch S3, switch S1, and finally switch S2 all reconfigure themselves to the configuration found in switch S4. As each switch reconfigures itself with VLANs that are not supported in the network, the ports no longer forward traffic from the computers because they are configured with VLANs that no longer exist on the newly reconfigured switches."
Why does this happen, if S4 was a client?