Why do frozen vegetables go bad more quickly after thawing than fresh cut ones? I left a pack of frozen vegetables (brocolli, carrot, cauliflower, green beans) in the fridge so it thawed. It went sour and slimy after a few days, (maybe 5) -- why?

If I cut the same fresh vegetables and store it in a plastic bag or container, it will be fine in a week or more.

What's the science behind that? Why do bacteria grow more quickly in frozen veggies being thawed than in fresh cut ones?
Very simply; freezing breaks the cell walls.  

As ice takes up more space than water, as the vegetables freeze all the cells rupture.
Once defrosted, each cell is then little more than a slowly-leaking bag of nutrient, exposed to the open air. Whether or not any bacteria gets in &amp; starts to breed is almost irrelevant by this time, as your 'leaky bag' is going to dissolve into a pool of its own juices pretty quickly.

This is why frozen veg is cook-from-frozen not cook-from-thawed. An hour might be OK, a day is starting to get a bit much. By 5 days, I wouldn't do anything other than move it to the recycle using tongs.