It's one of the idiosyncrasies on a mobile device. For whatever reason (on the iPad or the web site), if the coding to load the site is not finished, it will not recognize that the keyboard is ready to use. Unfortunately, there is no way to prevent this except to wait for the web page to finish loading.
You can, however, try to speed the process along: When the web site is loading, you will see an "X" in the unibar (next to the web site URL). Press the "X" and the web site will stop loading. Hopefully, then, you'll get no interference and your keyboard will display properly so you can use it.
Marilyn