Gantry Upgrade to C-Beam

I tried cutting the brackets in aluminum this weekend with Mixed results.
I broke the job up into lots of smaller operations so I could easily adjust if needed.

Most of the holes and countersink operations went well. Even the larger stepper motor mount hole.
I ran into problems when doing the final outline profile cut.

It cut fine until just as it was cutting through the bottom. It seemed to snag, then jam and broke the bit. My brand new 1 flute coated bit.
Ugh.
I looked it over, it looked line the bit may have snagged on a bit of bur as it broke through?

So I switched to my back up bit a 2 flute coated bit. And it was doing well, until it did the same thing again in a new spot.
This time it really knocked it out of alignment. I should be able to recover alignment enough by rehoming. Fortunately the final outline is mostly cosmetic so if it is slightly misaligned it will still work.

But I am unsure of what when wrong.
*It could be snagging in burs as it breaks through?
I can set up the cut file to run the last few passes at a much lower speed.
*It could be a build up of shavings in the cut?
I did vacuumed it out by hand several times during its cut but I can do more.
*I had tabs on only 2 sides, maybe it is shifting just a bit as it breaks free and binding the bit?
I will add a few more tables.