You would be surprised how the water can leak past the door seal, slip under the aluminum strip and work under the flooring then spread through the particle board in the flooring. If you are doing a remodel I recommend pulling every window and replacing the sealing as the factories do a poor job. Also check the bottom of outdoor speakers to ensure they have not rotted out allowing water in. All lights need resealed on the outside just like the windows.
I had the same general damage to the floor in the back corners of my unit. After replacing the flooring when I reinstalled the flooring I caulked the entire back and edges making it water tight on top. I haven't felt any soft flooring since then. I also had every event I spoke of in the first paragraph.
Good luck with your remodel!