Why is my wireless signal constantly switching to a non locked down signal (Mac)

It’s been happening all day. All of the sudden it seems that I’ve lost my Internet signal but when I check my network settings the signal has inexplicably switch to someone in the neighbourhood who hasn’t locked their signal down.

I have my signal locked down. The network is available, working and set as my default. Every time it happens I have reselect my wireless signal. What could be causing the switch?