A few reasons:
1. Barrel length. Could be you're just burning powder out past the muzzle.
2. The more common(and much more dangerous) reason?
You're powder's pissed. you've pushed your powder into pressure ranges it WASN'T designed for. You are getting to the area called "thin ice"
3. If your velocity comes in lower with a higher powder charge?
You are practically BEGGING for a disaster to happen.
The above results assume a long list of things:
Proper case prep.
Trimmed right?
Uniform crimp?
Primers seated properly?
Are cases from the same lot? Do they have the same internal volume?
Powder charge correct? Sure? Check it again anyway.
Chrono reading correctly?
Are you testing at the altitude, and with the weather conditions and temperatures in which you intend to use that particular load in?
I don't ask these questions to scare you out of testing the limits a bit. Just to make you very, very cautious.
I've been chrono mapping for years. But I'll share a little story that happened a couple years back:
Gun-
9.5" super redhawk in 44 mag
Brass-
Winchester
Powder-
Blue dot
Slug-
300 grain hard cast lead.
I wasn't happy with the speeds I was getting with 2400, AA9, VitN110, or 296. I ran out of case room before pressure signs showed or hitting the velocity I wanted. So I switched to a faster powder, blue dot.
All was going well in steps of .2 grain increases. Velocity and powder charge were going up even, no pressure signs. This went on .2 grains at a time for a full 2 grains.
The next .2 grain increase? Velocity dropped about 30 fps. And I had to tap the ejector rod with a small hammer to get the brass out. That powder(which is known for being tempermental) went from safe to redline in 1/5 of a grain increase!!
How accurate is your powder measure? Or do you hand weigh each charge?
Point being:
IF you push it-
Go slow, go careful, and don't ever do it without proper equipment or when you are destracted. Then make sure the loads you work up are used ONLY in the gun the load was developed in.