Apparently we humans have a "force field" around us which is difficult to penetrate. Nonotchka has found the switch to turn it off, but Notch (pictured here) hasn't.
The force field sets in rather suddenly. Notch might be perfectly comfortable sitting 15 inches away, munching happily on a walnut. But put a nut a little too close, and she goes into "pushing through the force field" mode.
Apparently it's easier to get through a force field if only the front part of your body has to penetrate it. If you can keep your hind legs outside the force field, it's much more comfortable. Also, when near or inside the force field, a tail is an excellent ward against danger: if you hold your tail between you and a dangerous enemy, they will find it much more difficult to attack you.