what causes this and what does it portend?
It's one standard solution to the two body problem. Other solutions are the parabola, hyperbola and circle. The parabola and circle are very unlikely since things have to be just right. Remember that a circle is just an ellipse with both foci at the same point. A parabola is an ellipse with one focus off at infinity and in a hyperbola (or pair of hyperbolas) the other focus has wrapped itself around infinity and shown back up in real space. Note that, in some rare cases, the orbit of the planet (or whatever) will intersect the surface of the primary. This is what happens when you throw a ball up into the air. The ball is in orbit around the Earth for the brief period between the time it leaves your hand and it is caught or otherwise stops being in free fall.
What it portends is that we'll keep going around the Sun till, parboly, the Sun goes Red Giant.
