I can offer a theory as to what they might be, based on what I’ve seen cutting acrylic…but it might or might not be what’s causing that.
Acrylic is a strange material to try to cut…it vaporizes in the laser beam, but the part that is left behind is very sensitive to high temp and it can go through some changes that will cause the paint on the back to “detach” slightly for a fraction of a second, leaving those marks in the mirrored finish that is applied to the back.
You’re going to notice them particularly on really thin cutout areas, or tight curves and angled switchbacks in a design, where the heat gets concentrated while the beam is moving.
I have a way to work with this kind of situation that is unorthodox, but it should work pretty well for something like this. Place cuts in your design at every corner. It will consist of open paths that just meet up at corners. Then give every other path a different stroke color. Like red - black - red - black…all the way around the design.
When you process the design, do it in two stages. Cut all the Red lines first. Set the Black strokes to Ignore in the thumbnail column.
Then wait a minute for the acrylic to cool.
Last, without touching anything else, change the Red paths to Ignore and send the Black paths to Cut.
Use the fastest speed you can to cut through…it keeps the heat down in an area.