I’ll respectfully disagree with the posters suggesting your z axis need to be squared (though certainly that would exacerbate the problem). These are dwell marks (google “cnc dwell marks”). The posters who have suggested deflection are correct. It’s caused by movement changes. Typically, the solution for dwell marks involves some combination of lead in/lead out and ramping. However, for your problem with tabs, the more common solution is 3D tabs (radial or triangular). With square tabs the bit dwells in one position while it plunges/retracts, allowing it to overcome the deflection and cutting more deeply into the wood, ultimately leaving a dwell mark. The solution involves either reducing deflection by (a) increasing rigidity (of the machine and/or by increasing the bit diameter), or (b) reducing feed (again, to reduce deflection); or - more realistically - avoiding any change in deflection by keeping the spindle in continuous motion though the use of 3D tabs.
Unfortunately, I don’t believe Easel has 3D tabs (don’t know, I don’t use it). I use Vectric software and for the most part completely eliminated my similar issues.