Hi & welcome! Sounds like you might have had fungus gnats as well as centipedes in the soil. Both inhabit soil, but don't bother plants unless/until the dead organic matter in a pot has been consumed. At that point, they could start eating roots. The process of eating the organic matter (and returning it in the form of poo) radically changes the soil structure, turning larger particles into "dirt." Once that process gets to a certain point, the roots can begin to suffocate & rot from lack of oxygen/increased water retention. I would repot your tree while the weather is still nice. If interested, you can find many discussions about FL fig in houseplant forum. Raid could be toxic to plants as well as critters...

How long have you had the tree? When did you last repot? Full repot, as in removal of all old soil, bare rooting and root pruning. what type of soil? how often do you water? What do you use to tell it’s time to water again? from the photos, it looks to be in a spot that doesn’t receive enough light. Hense the leggy growth. These are full sun loving trees naturally, so give it as much light as you can. Right up in front of a south facing window that’s unobstructed would be best. this is a good link to help understand Ficus trees. The main focus is on Ficus Benjamina, but your Ficus Lyrata (Fiddle Leaf Fig) has the exact same cultural needs. http://forums2.gardenweb.com/discussions/1476078/ficus-trees-in-containers-iv?n=223

Have a photo of the pot? Soil? Drain holes? Looks to be over or under watered, which will give the same response.

oh my god! this is not even my plant. i was just asking for the name of the scissors. i am not the owner of the fiddle leaf fig that has a rotten root. i had to cut the container because it is a little taller than the decorative pot. around 0.5 inch only. the container is white and the pot is black. for me it looks ugly so I want to cut the top part. why is it so bad? why am i the bad person here?