I had this happen a couple days ago while cutting 3/4" plywood with a 1/8" two flute bit. I had cut dozens of parts prior without issue.
I discovered four issues while investigating. #1 was I had clamped down one side of the board but forgot to clamp the other side.
#2, not only did I forget to clamp the other side but the board was on top of the clamp causing that corner to be 1/2" above the sacrificial board in that corner. Major brain fart on my part.
After taking care of 1 and 2, I tried a test cut using a 1/4" router bit that I’ve used in the past with no issues. I didn’t get the vibration but I did notice that the bit was all over the place and moving too fast. This led me to discover issues 3 and 4.
#3, I discovered a couple mm of free play in the center gantry. This wasn’t there previously so I’m not sure if the vibration caused an existing issue to become apparent. One of the lock nuts was barely in the nylon and the bearing wasn’t touching. I thought the PLA had cracked but it looks fine. I snugged up the nut and all is nice and tight again.
#4, the feedrate for the tool I use somehow went back to defaults which is a bit too fast for the size of my mpcnc.
After fixing issues 3 and 4, I’ve had no more problems.