This isn't going to be long, as it's a fairly easy observation (and doesn't require a single shred of maths!).
The New Eden wormhole system is a closed network. Put simply, at this stage of wormhole exploration, we know that there are a finite number of systems that are reachable by wormholes. These generally receive new connections every few hours... quite a regular event for a natural cosmological phenomena.
What's more, is that the locations of these wormhole systems, whilst we cannot tell their distance or relation to the populated systems, are all generally in the same parts of space. This again, is a fairly simple observation to make: looking out at the surrounding nebulae of wormhole systems we can see that they share features and colour spectra.
Knowing this, we can deduce a few possibilities.
Locations of natural wormhole viability
It could be argued that these areas of space are places where wormholes are particularly viable. Perhaps something to do with the presence of large mass features (pulsars, black holes etc.), these just may be locations where space-time more readily folds in on itself,
It can then be argued that the Seyllin event led to the New Eden cluster becoming suddenly more susceptible to wormhole formation.The exact mechanism of this is definitely something that deserves more research, and means that wormholes are more likely to be natural phenomena.
Artificial wormhole network
A somewhat more extraordinary possibility would be that the wormholes we experience in New Eden are in fact constructed. Much like our own jump gate network, each system connected to wormholes already appear to be inhabited.
We don't find many completely empty systems... and in fact there are some systems in New Eden which are even less populated than wormholes. A short trip through even High security empire space will show you transit systems, with some asteroid belts, temporary anomalies and a pair of jump gates. In comparison, wormhole systems appear to be stuffed full of Sleepers.
This hints that wormholes are actually a network to settled systems. If they are artificial, then it would explain why there are some features of wormholes not predicted by relativity. Or, you know, relativity is wrong.
The middle ground
Odds are that the answer lies somewhere in the middle of these two extremes. The Sleepers created a technology allowing them to create wormholes, and those more densely inhabited systems (Class 6s) are simply systems where exit wormholes form more readily.
If New Eden was not originally a place where wormholes formed easily, then it's reasonable to assume the Sleepers left the cluster, and simply closed the doors behind them. The Seyllin event, as destructive as it was, re-opened those doors.
What might be more relevant is that the wormhole network became unstable. We have seen over the years the number of wormhole connections across New Eden increasing. Low sec connections recently increased, and some scientists have predicted Null-sec systems are about to be affected in the same way.
This increasing destabilization of the wormhole network is already opening up new systems previously inaccessible to capsuleers. It may be that there are some systems that were also locked away by Sleepers... systems housing dangerous, or criminal Sleeper factions.
Maybe what we know as Drifters?
Pure speculation, of course. But something we can start looking for; structures that control wormholes for a start. But something even better, would be structures, or destroyed structures designed to inhibit wormhole creation. The Sleeper equivalent of a Cyno inhibitor.
Evidence to look for:
Wormhole control structures (working or wrecked)
Common features between newly opened wormhole systems
Maps and terminal stations
Maps and terminal stations
Additional: I can't find the dev blogs or forum posts for changes to wormhole spawn rates in low and null sec. I've seen other writers talk about them, but I can't find actual evidence of it. I'd appreciate someone find me the links, to prove to myself that I didn't just dream them.