Why do my coworkers all of sudden try to up one me, now that I’m voluntarily doing physical labor and working overtime?

I don’t get it, it’s obvious my coworkers don’t like me because I’m doing more. But what confuses me is whenever I clock in to work, my coworkers either try to race me to the tasks or work longer hours just as myself. But on my days off, I have noticed that my coworkers don’t do much of the tasks and my boss gets upset about it. That whenever I voluntarily step up, so do my coworkers. Why?

Why do my coworkers all of sudden try to up one me, now that I’m voluntarily doing physical labor and working overtime?
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