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Tarps sag like that, especially if they get wet. The seam around the edges has the fabric folded a few times, so it's stronger there and less likely to stretch a little bit at that edge. In the center of the tarp, you only have one thickness of fabric and it's gonna stretch just a little. Over the length of a long tarp, it's magnified. When it stretches in the middle, but not at the edges, you get a sag.
One thing I like to do is have a line stretched across the site from tree to tree, pretty tight, and throw the tarp over that line so the line supports it in the middle. Like the ridge beam on a house. You still get some sagging on either side of that line, but it's less than without it. Tightening up the lines at the corners can help get rid of a sag too. But anyone who has been camping in the rain has grabbed a long stick to prop it up in the middle.
In scouting, the boys like to push up on that puddle of water on the tarp as somebody is walking by so it dumps on them.
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