Two main possibilities.
One, the app you are using to read is buggy. If it is hte most recent version, all you can really do is wait for a bug fix update. Or, if the content allows switch to a better app.
Two, the iPad's OS is glitching. Many possible reasons for this, but the most common fix is to power the iPad off, and then turn it back on again. If you have not done so recently, give it a try.