When a prints corners and sides curve upwards, it is refered to as warping. It is one of the most common problems when printing in materials with a high shrinkage rate. As polymers cool, they shrink, and if the ambient temperature is not consistant, the print will shrink unevenly. Even when parts are stuck to the bed entirely, there will often be delamination of layers as a result of warping.
As mentioned above, warping is caused by uneven cooling of the print. The larger the part and the higher the infill, the higher the chance that the print will cool unevenly and either delaminate or warp, since the center will be hotter than the surrounding areas. The mere fact that the build plate is heated to a temperature hotter than the ambient air will always make warping a possiblity, even in enclosed environments. This is why it is imporant to know your surroundings: if printing in very cold environments, warping will be a much higher probability.
The material being used is also a huge factor in the amount of warping that can occur. PLA has a very small shrinkage rate, so long as the temperature is not extremely cold around the area, and a heated build plate is used, the print should not warp. ABS actually has a very high shrinkage rate, and high glass transition temperature, which both mean ABS is very susceptible to warping.
Some specialty filaments are even more likely to warp, making it very crucial to understand the material being used.
Warping may not be entirely identified until hours after the print has started. Unlike an unlevel bed, this may take your printer offline for half of a day and waste a lot of material.
Taking tips from the Bed Adhesion section will go a long way. If you do not have a enclosure for your printer, you will need to understand your alternatives to high probability warping materials. There are materials with similar strength properties as ABS such as PETG and PET+, that do not have such a high liklihood of warping. These can often be easier to print, though they have a lower glass transition temperatures and different elongation properties. Preparing your build plate and planning ahead with the right materials can go a long way in saving hours of wasted printer time and frustration.