Paper curl is a fact of life with inkjet papers, especially with fine art hand made papers and papers that have come off a roll. All paper ultimately comes of a roll - it is either cut into a smaller roll from a larger one or cut from a roll into paper sheets.
Further, due to the grain of the paper (the direction in which most of the paper fibres run), and the type of coatings used (which will react differently to the paper base to changes in temperature and humidity), paper is destined to curl. It's part and parcel of the entire production process and not really a fault per se, more something that is a fundamental part of the natural paper making process.
If you really hate paper curl, then you have a few choices. Double sided papers, being coated on both sides, tend to curl less. Museo Portfolio Rag is not really a double sided paper but they've treated the back side to help the paper resist curl. In general, resin coated papers curl less than fibre based ones as the resin helps the fibres resist the urge to curl.
But largely, if you want to use beautiful fine art papers, paper curl is a fact of life you are bound to encounter. Here are a few tips for working with paper curl.