The fact that they became disorganized after the destruction of the Lucifer is hard canon.
Where? The tech database is the only place I remember any mention of it at all(outside ST:R), and I believe it's quoted on the
Shivans FSWiki page.
A leading hypothesis is the hive mind theory, arguing that Shivan society is broken down in specialized functions driven by a collective intelligence. The most convincing evidence supporting this theory is the behavior of Shivan forces following the destruction of the SD Lucifer, the turning point of the Great War. Other experts caution against attributing insectoid properties to the Shivans, regardless of their appearance and behavior.
Nothing explicit; they reacted in some way, though I assume we're just supposed to connect the dots and assume they either went berserker/nuts or suddenly became stupid(since a major part of their theoretical "superbrain" just died), but nothing about canon ST seemed to suggest that.
Running over the
Command Briefings for ST, and the best I recall, the Shivans:
1. Attacked you protecting the Einstein's escape pods(IIRC, including the use of Seraphim bombers, which admittedly is a little weird).
2. "decimated our forces in the Ikeya system. The enemy annihilated our installations and eighty percent of the fleet garrisoned in that sector", and disabled a science cruiser(the Ratna) carrying "cutting-edge weapon protoypes".
3. Still had a formidable "primary Shivan fleet", which the Hellfire cruiser group was attempting to rejoin and reinforce.
4. Sent Seraphim bombers(and I believe some fighters) to attack the already-crippled Talus Mining Facility, which was also distracted by pirate attack.
5. Sent yet more fighters and bombers to attack an ultra-classified convoy(Iota), though to be fair, all they'd really have to do is distract you from shooting the insane homing asteroids.
6. Attacked or were attacked by GTI operations in Ross 128, with a Lilith and fighters, giving a pretty decent showing for itself against two Leviathans and their escort.
7. Still had a Cain to spare to guard a wing of freighters in next to last mission of ST, on a route with "heavy Shivan traffic".
Doesn't sound like they were all that brutallly crushed by the Lucifer's loss, or they adapted quickly enough.