I tend to transplant all brassicas deeper than they were in their containers and hill slightly when I'm hoeing around them. My soil is quite heavy and I firm them in well when transplanting, so that provides stability as well. I would think your plants are just fine; that's how the plant looks at ground level, which is why I bury deeper for support. Nothing wrong with staking them if they need it. Next time you'll know. :)

I remember my father saying he planted in 'firm' soil as they prefer to be firmly planted, which would prevent your problem.
So I tread the soil down before I plant any brassicas, deeper than they were growing.

I have mine staked because they're gowing in compost and mulched leaves, not soil. Not quite enough for the roots to grab on to.

A few of my broccoli plants fell over this year after a wind gust around 50-60 miles per hour. I did not do anything to them and like all plants, they grew back upright without me having to do anything. I did have one plant snap in half though. I keep the soil firm around mine; I always walk on the ground right up to the plants.

Thanks for your voices of experience. I was afraid something was really wrong. I'll try firming the soil around the plants, and will stake where I have to. I grow Dahlias, so I have plenty of stakes.