That's about how I ride most of the time on the trail. I wouldn't advise roping like that, though. Still, a lot of folks cinch way past what is necessary. My own mother, being the worrier in the family (that's her job) nearly ruined a good horse like that. It started acting just the way the OP's horse is acting. Once my mother pretty much gave up riding, the horse became my trail horse. I ride with the cinch snug, but not tight. Took me many years to get the horse over the cinchiness. We had a few go-rounds while we worked at it.
Hmm, just brought up another thought along the same lines as far as root cause. The main reason why saddles slip around on folks is because they don't fit. So, they may just think that the cinch is too loose and needs to be tighter when it's actually the saddle
at fault. That could contribute to the cinchy issue as well because if they're screwing down a saddle that's already pinching, I'd get pissed in a hurry too.
Of course, horses who are mutton withered, the saddle will slide a bit when mounting just because there's no withers to keep it in place, but on a horse with decent withers and a saddle that fits, any decent horseman who's in decent shape should
be able to get on without even having a cinch on the saddle.
decent rider, regardless of their size, should be able to ride without a cinch on a horse with a well-fitting saddle. Except, of course, in high torque disciplines that require fast turns or hard stops. Average pleasure riding though? If the saddle's slipping around, then either the rider has the absolute worst balance in the world or the saddle doesn't even come close to fitting.