Mechanics and Fixing Any Questions Answered

Sorry to ask a repeat question, but this is really bugging me because the problem reoccurred today and I feel it is only a matter of time before it happens when a car is just behind; opinions would be really welcome (or if this is the wrong thread to ask in, do let me know where I should go!)

The first incident was when I was commuting to work and all of a sudden heard a bang, like as if my wheel had gone into a pothole or my pedal had scraped the floor. But neither of those seem to be it because there were no significant potholes on the road and I was going straight.

My saddle had been turned 90deg sideways under me (like as if the bike had suddenly fishtailed while I was still seated, with the seatpost just slightly loose enough to move given sufficient force) but I thankfully remained on the bike and guided it to a stop. I found that my rear wheel was slightly mis-aligned and hence pressed into one side of my brake calipers, but when I shifted the caliper and spun the wheel to check, it spun freely and didn't seem out of true in relation to my chainstay.

Turned the saddle back round, got back on the bike and cycled the last 5 minutes to work completely uneventfully - and made the commute home absolutely fine like as if nothing was wrong, at the end of the day.

Happened again when I was going on a straight road, heard a bang/pop and fishtailed, came unclipped, rode my top tube for a while before braking the bike to a stop. Checked the rear end, no diagnosable issues.

Just today I was making a slow turn onto a road from a pavement, again heard a pop like as if my rear wheel had gotten jammed, and came off the saddle onto the top tube. Went into the wrong side of the lane but thankfully it was a quiet road so the oncoming car had plenty of time to see me and slow. Got back on the bike, checked the rear wheel was spinning freely, then made the rest of the commute just fine.

Pics of the rear end attached; it’s a fixed gear conversion so I have 0 idea what could have happened, because:

• at first I thought it might have been the mudguard getting jammed in the wheel, but I can’t seem to replicate the issue (the mudguard flap coming in just rubs the wheel rather than jams it) and there is a good amount of clearance, not to mention it should have popped off if it was really jammed given it’s only held on by a rubber strap
  
• Nuts are done up fine and chain tension remains normal and wheel aligned (at least as of the second and third times) so that does away with the wheel slipping in dropouts.
  
• Spoke tension seems fine
  
• Chain was changed less than 1000km ago and cog hasn’t seen more than 300k
  
• lock ring on cog seems nice and tight
  
• No visible cracks on any of the tubes, though I don’t know what I’m looking for..