Well, I've been going over it, and this is the best I can figure:
They're leftovers.
Most of the battles are for specific nodes, especially those between the two spaces (Antares, Vega and possibly Regulus, not sure on that one). However, odds are good that over time, and there's a lot to play with, bits and pieces of each fleet got stranded on the other side. Instead of wasting their lives trying to get back through the node and home, they wandered deeper into enemy territory and began blowing things up, especially transports and freighters.
After awhile, it probably became a viable tactic, and the Vasudan (possibly the Terrans as well, hard to say) began pushing their forces farther and farther behind enemy lines, until they got as deep as Ross 128. Most of this was probably made easier due to nodes behind the lines (save probably Sol's) were pretty much unpatrolled, or lightly guarded. The Galatea was probably sent to Betelgeuse to clean up one of these pockets at the beginning of the game. Meanwhile, the main fleet was busy holding the line, trying to keep the Vasudans from pouring in while trying to go "over the top" themselves.
So the war (near the end anyway) was a trench battle, but with lots of little battles throughout both sides territories. There also seems to be a great deal of ground combat as the losses on both sides is not only high, but attrociously high, far greater than fleet actions could possibly do. There is also reports of large scale food shortages on each side, which either means extensive ground warfare, or the cutting of supply ships (meaning those little battles behind the lines are very effective and used by both sides).
That's my thoughts on it at least.