That is just a cover over the plastic underneath. Before you go to drastic measures I suggest just pulling off that outer cover and see what is behind it. I suspect something got behind it and is pushing it out. Otherwise there is nothing that I am aware of there that can cause it to do that.
If you remove the 4 tie downs and all of the plastic rivets (The center piece pops out then the whole piece will pop out) this section will pop right off. Can't remember if the rain gutters are connected to the inner piece or the piece you are talking about.
When pulling be careful because once you remove the metal tiedowns the side panels behind that cover panel are then only held in place with a small screw that holds the saddle bag latch to the side of the truck. To remove the whole side panel also requires removing the sail panels. But you should be able to remove the cover that is giving you the issues without removing the whole side panels or the sail panels...
If you don't know what I am talking about let me know...
But again, the plastic piece you are talking about just clips to the actual side panel. The actual side panel is also plastic. I suspect if you pull it apart you will find someone carried a load of sand or something and a bunch of it got behind the panel and has worked it to the point it put pressure on it and now you are seeing separation. Won't know for sure until you actually pull that cover off.
When i install my amplifiers in the saddle bags I expect to leave these covers intact to cover the access to the amplifiers...
For more information check out my thread here and it should explain things better and show you what you are getting into...
https://www.chevyavalanchefanclub.com/cafcna/index.php?topic=142196.msg2119090#msg2119090
Do NOT use any sort of screws... Just fix the problem... Hopefully with the thread around this message you will understand what you are in for better.
Rodney