the issue is known to occur with some third-party dual-band WiFI routers, and tends to happen if you're the same name for each network or different security settings for each network. To ensure your iPad automatically rejoins WiFi networks, Apple suggests (you guessed it) giving the networks different names, and making sure that they're using the same security settings. Of course, that doesn't explain why other devices are able to work just fine with said routers, but this looks to be the most we'll get out of Apple for the time being.