...But Not Too Strong

I've had a curious strength-related challenge recently: men who believe I'm not physically strong enough to lift the weight that I do.
This isn't a problem in the gym, where I've received praise and encouragement to lift more weight, and more regularly. There, I deadlift 230-238 pounds for repetitions. No, this is taking place at a part-time job I'm doing to earn some extra money.
I work in a food factory--perhaps better known as a production bakery. Part of the job--I thought--was to empty yellow trash can-sized containers of spilled "product" into a larger container. I had no problem doing this--the yellow containers hold about 60-120 pounds of product; using appropriate lifting technique (tight core and bent knees), I'd empty them with no problem.
Well, it turns out--there was a problem: some men saw me do this a few times, became alarmed, and reported it to my boss. One day, out of the blue, he told me not to lift the containers, that he would take care of it. I protested, saying that I could easily lift them (and I can), but he was adamant.
I couldn't figure out why, however. It's part of the job--but not if you're a woman, it seems.
And the other day, I lifted several boxes (not all at once!) of packaged product, while helping out on the assembly line. They each weighed about 55-70 pounds, and I (again) lifted them with no problem. Well, a few minutes later, I heard some guys talking about it. The next thing I know, when people on the line asked for a box of product, these same men nearly knocked me out of the way to get to the boxes, preventing me from getting near them.
Is this about men feeling invalidated if a woman is strong? I have no idea, but it is frustrating, to say the least