There has been a large attack on Sol by players pledged to Powers other than Jerome Archer, which exceeded the ability of Archer-pledged players to reinforce against it.
So far no-one has claimed responsibility for doing so, which is somewhat unusual - successful undermining is so rare and so counter to the design of Powerplay that normally if someone manages to do it they want to brag. But it could have been a group outside of the obvious channels - a group of friends who all know each other, or maybe a language barrier. Or they might be saving that up for later. Either way, this means that no-one outside that group knows precisely how it was done [1].
In theory any sufficiently large and active group of players could do this to any Stronghold system at any time - Sol has no special protection in this respect: Archer's HQ which is protected is the Nanomam system.
In Powerplay terms, there is no special formal significance to any system - nothing "special" would happen if Archon Delaine captured Achenar or Zemina Torval captured Alioth - it'd just be +/-1 on a couple of Power's total system counters. Obviously Archer losing Sol to an Imperial power would be surprising and potentially a bit embarrassing, but in Powerplay terms they could just capture and reinforce some other random Col 285 system elsewhere and come out even on the Powerplay leaderboard.
[1] For completeness I should note that therefore, because successful undermining is incredibly rare (Frontier semi-accidentally designed Powerplay to encourage 12-way truces and peaceful reinforcement), many Powerplayers are expressing suspicion that some sort of exploit was used. There is no direct evidence for this, it's just the normal conclusion when systems actually get undermined because in the current design that's not "supposed" to happen.
