What's changing the default route?

I use an ssh tunnel for a VPN on OpenBSD 6.1. To initiate the VPN
connection, I type the following on the local machine

# ssh -f -w 0:1 R true
# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
# route add R G
# route change default 10.1.1.2

where R is the IP address of the remote server, and where G is the IP
address of the local gateway.

Usually, this works well. But I occasionally have a problem where the
default address in the routing table will spontaneously change from
10.1.1.2 to G. What could be causing the routing table to spontaneously
change in this manner without my intervention?