Also be sure you do not mix different materials like light weight clothing with towels. Towels hold more water so when spinning they will pull the load even if clothing is visually distributed.
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=== Update (01/22/2018) ===
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Update:
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On two occasions I found a worn out belt to be the cause. On my machine it was visibly worn, but later, on a customers machine, it looked in great condition. She always does light loads so I could not fathom why that was going off balance. After scrutinizing internals, I could only think of the belt as a possible suspect. After changing it, all was right as rain. I do not pretend to know how the belt causes this but it just works.
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On another client it turned out that the agitator was slightly bent! It was one of those machines that locks you out, so she never noticed that when she bought it. It worked iffy from day one so she attributed it to being new to washing machines and old house flooring. Two years later it was growingly annoying to just wash. After separating towels it made it bearable. At least until the machine completely conks out.
Also be sure you do not mix different materials like light weight clothing with towels. Towels hold more water so when spinning they will pull the load even if clothing is visually distributed.