I think it's just to be comfortable.
Let me explain. I go to Starbucks nearly every day. The Starbucks I go to recently remodeled and changed their "comfy chairs." The old chairs were plush and the seat was farther from the floor. The new chairs (see picture) have their seats lower to the floor. While sitting in the new chairs, I find myself "manspreading." Why? Because my legs are too long to sit with my feet flat on the floor and my legs together. This isn't comfortable. So I either spread my legs or put my feet out with just the heels of my feet on the floor. Either way, it's not to assert dominance, it's to be comfortable. And a lot of chairs, such as in subways, buses, etc. have low chairs that are comfortable for most women and shorter people, but too low for most men.
Woman tend to have shorter legs. My wife says she likes these new chairs because her feet touch the floor. So women don't need to spread their legs to be comfortable, generally. And yes, there are cultural taboos against women having their legs spread in public for any reason.
But I think it's mostly chair design, not some cultural plot for male dominance.
So there ya go: manspreading mansplained.
Agree? Disagree? Think I'm nuts? Let me know in the comments below.