I've done so more research on this and it appears this is "normal". As long as the center is not sinking (which would be rather hard to do unless you have some REALLY soft soil / sand and didn't compact it at all). If the center isn't sinking, just keep going around and around the pool pulling the legs out keeping them from buckling. Once the water reaches close to the top, the sides will go back to place straight.
Also, one thing I noticed is you don't appear to have pavers under your legs, unless you buried them under the sand / dirt. I would STRONGLY recommend you have pavers. If you don't, stop filling now and get some under there. Once the pool is full, you won't be able to lift the legs to put some under them, plus it will change your levelness of the pool, unless you countersink the pavers into the ground. If you don't put pavers, once that soil around the legs gets wet, they will start to sink like rocks. Once they sink, you will have issues with the pool staying level. Once you have issues with the pool staying level, you will have issues with the integrity of the pool ... I.E. it may will eventually collapse completely.
I will try to find an AG owner that has the rectangle pool here and see if they can provide more insight. Confirmed. These pools all but fall in on themselves while filling. Just make sure you are LEVEL when starting and stay LEVEL and keep pulling the legs out. Don't forget those pavers!