Going from memory, that would be due to my internet going down, at which point kalle’s miner (“signet-1”) took over. The fork became visible when I went online again with a longer/more-work chain, but at that point I manually invalidated my chain so that the chain that had been public would win out.
There’s GitHub - ajtowns/stale-blocks-signet · GitHub tracking some of the recent stale blocks.
Regular reorgs have long been intended on signet, see for example:
I haven’t done anything to try to make deep reorgs happen on any regular basis; I think with my current setup seeming to be working okay now, I could possibly make deeper ones happen automatically too. Depends on if there’s a desire to be able to force double-spends to occur as well; that might be trickier to automate sensibly.