How closed is the return pipe? I just had this problem with mine and it turns out, it was due to not enough air. Let me explain.......
Over the last few weeks, my skimmer would overflow for no apparent reason. I would open the overflow tube a bit and that would take care of it for a day or two. Again and again this happens every day or two until the pipe was 100% open. I finally pulled my pumps and cleaned them, guess what, the air intakes were getting clogged. This was causing more water to be pulled in, causing the overflowing. This increase in water flow was also causing bubbles to be pushed out of the skimmer.
Check your air intake muffler, tubes, and intake at the volute if your pipe is open. If the skimmer has been down for some time, give it some break in time to slime coat everything.