Does this happen in all places or just a specific WiFi? If it is just a specific one, the problem may be the WiFi itself. Try rebooting the router/modem or even check with the ISP to see if it the connection is going out often.
If it happens on all WiFis then try the following...
Well there could be a few reasons. First I would make sure that there are no apps that were recently installed, or updated, that could be causing it to disconnect. Either through just messing with the connection (yes some are just made to annoy) or by trying to do things through the WiFi and it keeps kicking it out. So I would check the device.
If that isn't it, then sometimes you need to adjust the WiFi sleep policy on the tablet. It can often cause this...