Why did my humidifier water bottles get discolored inside?

I’ve had a stand alone type of whole house humidifier for many years. It has two 2.75 gallon white plastic water bottles. In addition to using an anti-bacterial with each fill-up, at the end of each winter I put some water in each bottle, add a little bleach and shake it several times. Then I rinse the bottles out, get as much water out as I can through the small openings and let them sit open for 1-2 weeks to thoroughly dry. Finally, I put the caps back on and store the unit away.

Yesterday, when I took it out to set up, both water bottles have large areas of brown discoloration inside as shown in the photo. I don’t think it’s mold. Could it be deterioration of the plastic? I’ll be getting new bottles anyhow, but would like to know what happened to prevent a recurence.