I've heard occasional mentions of this happening in extremely busy systems - once a very long time ago when Quince was the goldrush, at the Gnosis, at the first Void Opal goldrush before people realised there were hundreds of suitable systems, and then this weekend. The first two of those of course pre-date the system-wide multi-mode chat.
The most obvious explanation is user error, of course.
Still, the connection between the places it's been reported does suggest another possibility - we know that the modes are basically just an instancing filter, so what if there's a maximum number of instances per location or per system [1]? A really high number, so for the first few years of the game even big group events didn't get close to it, but at some point it still gets hit and then Solo players start getting stuffed into existing instances because it literally can't create more.
[1] Whether an actual coded-in one, or a consequence of increasing load on the instancing servers.