A built up edge is when material attaches to the cutting edge of the tool. How it occurs is not clear. Some sources say the material welds onto the tip; others look at it as micro-deposition of material. However it occurs, a layer of the material being cut is deposited onto the cutting edge. This not only alters the sharpness of the tool; it also reduces back rake angles. When this occurs, chip clearance is reduced and temperatures climb even higher at the point of cut, causing more and more material to accumulate. This can cause significant finish defects. Now, if that BUE suddenly breaks off then the surface defect resolves and the cutter will function again provided the BUE didn't take a chunk of the insert with it like it usually does. This is what I think happened to your cut.
Causes of BUE are too low a cutting speed, too high a feed rate, lack of coolant (ineffective heat transfer) and changes to the tip geometry of the cutter as the BUE occurs. This happens commonly with aluminum but I've also seen it with steel alloys that can work harden.
So, clearly you can increase speed, reduce feed and use coolant. If a BUE occurs, change or revolve the insert. You can also reduce your depth of cut if you cannot increase speed enough to meet the speed requirements of the insert.
Who knew flycutting could be such an adventure, eh?