All apps should update, eventually, however it's perfectly normal to open the App Store and see apps waiting to update; since updates may become available at any time of the day. Normally the iPad only initiates automatic updates when the iPad is plugged into power and has wi-fi available. Even then it will wait a bit go make sure you are not using the device. Late at night seems to be the preferred time to update.
I have seen it update apps in the middle of the day and not plugged in, but (I suspect) this only happens if you got plenty of battery and a wi-fi connection, and if the apps that need updating are small.
It also likes to update just because you opened the App Store and looked at the Updates tab. Mine does anyway.