I think the key to remember is that bug exploration is usually extremely slow, and that they are more than willing to wait hundreds of years to expand again. Personally, I think they paused exploration just past Harnah for security reasons, then started exploring again after the fighting had started. They may not have explored there till they did because the closest Home Hives (4 and 5) were focused on the Justin campaign. Once that one basically came to a stand still, then they found Centauri. Here is a compilation of how I think the Bugs expanded from the begining. I know it's a little long, but I think it could be pretty accurate. It also shows where in their exploration I think they met the Star Union. It uses the system designations that are on the map in the front of The Shiva Option hardcover.
2. Bug Exploration
This really comes down to trying to figure out which of the Home Hives came first. I believe, based on its location, that Home Hive 3 was the original Bug home world. Expansion would then have gone something like this. Exploration of Bug-1, 2, 3, and 4 as well as Orpheus-3 would have happened about the same time. After that Bug 5, Orpheus 2, and Home Hive 1 are explored. Home Hive 1 is colonized at this time. For security, the systems immediately around Home Hive 1 are explored. Orpheus-1 and 2, Pesthouse, and Bug 10 are found. They now know of two paths from HH1 to HH3. Exploration now continues since HH1 is now built up. The Bug-5 to Pesthouse link is found, Bug-17, 7, 11, and 12 are found, as well as HH2. Home Hive 2 is now colonized. To protect HH2, Bug 6 and 8 are discovered. Exploration now continues with Bug-16, 21, and 9, BH-32, Shanak, and Franos. At this time, the first war with the Star Union occurs. The Bugs conquer Franos and Telik, and discover Bug 13 and 14 and AP-5. They start to get cagy about other races and decide to picket all systems and not expand past closed warp points until they need to. AP-5 is the only place they need to worry about this at this point due to Shanak not having any other warp points (or so they think), however, the ground work for this idea is set at this time. The next exploration discovers HH5, Harnah, and HH4. HH4 and 5 are colonized and B-18, 20 and 22 are explored for HH security. All exploration stops while HH4 and 5 and built up to full power. New exploration finds B-19, 15, and 23. This is the point where Commodore Braun comes from Indra and finds Bug-20 and HH5. The Fleet is mobilized through HH5 and advances all the way to Justin. HH4 starts to explore more to find more contact with this enemy. Bug 25 is found, but exploration stops due to the closed warp point. Contact is made with the Orions at Shanak and the Kliean campaign begins. HH3 is found from Zephrain, but the bugs don’t know it. Centauri and B-24 are found from B-15. Due to close proximity of the Enemy, further exploration from B-24 finds the connection to B-25. As Operation Pesthouse is underway, Survey Flotilla 19 flees unknowingly towards the Star Union, allowing the bugs to explore B-26, 27, 28, 29, and Pajzomo. The new war with the Star Union now commences, allowing the bugs to explore portions of Star Union space. The System Which Must Be Concealed is found now due to the food source that was an Enemy for a very short time coming out of its closed warp point into HH2. At this point, the bug exploration stops, as defeat after defeat takes systems away, until all they have left is the System Which Must Be Concealed.
Up until contact with Commodore Braun, all of these expansions take generations, with the exception of the first war with the Star Union. In many ways, the war, and the perceived need to push so quickly into enemy space is really the downfall of the bugs. Imagine how much more devastating that reserve would have been if the Bugs hadn't thrown it away for such little return in the Justin campaign.
