If I had to hazard a guess, Victoria's Secret models would be used to criticisms about how to do their job (need to be/look perfect), and know how to take instruction. I assume Fox didn't take instruction well.
Yeah, that's pretty much it. Bay's direction to both actresses was almost entirely based around how he wanted them to pose on camera - arch their back, pout their lips, that kind of thing - with almost no input on actual performance beyond that. Megan Fox became uncomfortable with that, while RHW is used to that sort of direction from her Victoria's Secret work.