When water on kitchen sink is turned up, the water flow gets excessively choppy
I'm still not certain why exactly this happened, but removing the filter fixed the thumping/choppy problem, and replacing the aerator fixed the second gust of water problem.

My suspicion at this point is that the filter either just spontaneously failed, or something in the water conditions around the time my neighbor had his problem caused a buildup of debris or something like that. We hadn't replaced the filter cartridge in it for some time anyhow (my wife found that it didn't seem to affect the water's taste much), so it was not a huge loss to us, just a baffling mystery at the time.