I just reviewed the logs from March 9th, the day I was closing channels.
All the problematic channels were on my node edge-1, and almost all of them are now closed, but one channel still remains open (c11795b2b5e4ab295a4055ce387345341547d3880208cf6499a9352672f86e90:0).
I started examining the logs at that time and encountered the following error (lnd): "unable to gracefully close channel while peer is offline (try force closing it instead): channel link not found"
I believe it was some sort of desynchronization during peer-to-peer communication.