Welcome to Android Central! So is it rotating to landscape when you're on a homescreen? Or is it rotating to landscape when you're actually in an app?
If it's rotating to landscape when you're on a homescreen, I doubt an app is causing it--it may have to do more with a sensitive orientation sensor. If you tilt the tablet a little bit just before placing it on the table, it might trigger the rotation, and then when you place it on the table flat, it will retain that landscape rotation. Other potential causes include an uneven table or a tablet case that might be making the tablet lay unevenly.
One easy workaround would be to toggle the Auto-rotate off right before placing it on a table. Just swipe down from the top right of the screen to access the Quick Settings and tap the Auto-rotate button to toggle it on or off.