Why does port scanning from within a network cause the network to crawl?

Port scanning is a heavy task for a network since the dramatic increase in requests that occur.
For every ping/request sent the remote computer needs to take time to answer the question.
This is the idea of DoS attacks (Denial of Service). Then huge amount of requests are sent to one host. Pings/requests are high priority and the host get busy answering requests.
During this answer process it is unable to handle regular traffic and are unavailable. It might even crash.