Why does the AI Spam Certain Units? Awakening of the Rebellion 

I get the impression the spam gets especially noticeable in these types of scenarios where the AI has a low level space station and no advanced shipyards; The AI is trying to build a fleet of a certain strength to attack the target, but because their options are limited to the low power-per-unit options they need to build a ton of them to compensate.

I've been playing "Jump to Hyperspace" as the Empire and I noticed that the Black Sun is a lot more aggressive with their research / attacks than the Rebels.

If the AI is building a force specifically to attack one of your planets, it will attempt to bring units that are good against your forces there. Units built this way also my sway to the generally high combat power ones per pop. That generally explains Braha'tok swarms. The AI sees them as generically powerful units. Definitely more powerful squadrons then T-Wings and Z-95. Please also note that the AI still needs advanced shipyards to build advanced units.

I'd guess they don't have any global research which would unlock more from lvl1 stations?

This also hints at certain viable strategies for the Rebels and Black Sun. If you target production facilities in a raid and then retreat you can still deal a heavy blow even with a small force; imagine sending in a handful of carriers with bombers to snipe a structure, or an operative to blow something up, then leaving, that kind of thing. Tactical loss, but strategic victory.