Too much water in crappy soil - your plant will eventually grow out of it, as it sheds old leaves and grows new, when it starts growing again. Get some of The Better-Draining Mix and repot it, taking time to get rid of the old, dead roots (the healthy ones are yellow-goldish) too as you get rid of the old soil. After you repot it, wait at least several days before you give it water (a drink, not a flood). Water 2x a month or so for the next several months.

Thanks a lot. Very helpful!

9 years later lol...

For future reference for anyone who ends up here searching for answers...

Check your plant for bugs, they will likely be at the base of where the leaves meet and further into the soul. Left untreated it will kill your plant.

Seclude your plant from others so it doesn't spread, use rubbing alcohol with a dab of fish soap in a sprayer or dab plant with alcohol. You can use Neem oil as well for treatment and prevention. This is the beginning of infestation.

Hope this helps.