Why would a local network unit's ip number change?

[joe at actionline.com]

I had rsync working fine between two computers on my local network, but
then it just quit working even though I had made no changes in either
computer.

This is the error message I saw:
ssh: connect to host 192.168.0.73 port 22: No route to host
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: error in rsync protocol data stream (code 12) at io.c(601)
[sender=3.0.7]
ssh: connect to host 192.168.0.73 port 22: No route to host
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: unexplained error (code 255) at io.c(601) [sender=3.0.7]

Upon investigating, I found that the ip number on the "73" computer was
changed from 192.168.0.73 to 192.168.1.106.  I tried changing the
destination address to "1.106" on the sending computer and that also did
not work.  So I rebooted the destination computer and found that the ip
number was restored to "0.73" again.

What would cause the ip number to have changed like that and how can I
prevent that from occurring again in the future?