I doubt that it care from a contaminated water supply. Sure, it's possible, but I think you'd probably know your water supply was questionable enough that you really should get a water tower or something, so you can do some basic control, like chlorination, or even just a sand filter and solar heating. You run the water into a big black pipe, then empty it when it reaches high enough to kill most bacteria, and you're as good as if you boiled it.

I think it's far more likely that they used "fertilizer" before harvest, without composting it first. I also think it's likely they didn't wash it properly.

I think it's likely that there was no oversight into the operations of whatever farm the lettuce came from. They probably bought it from some random farm with a dirty operation that contaminated the lettuce due to simple stupidity.

Of course, we won't find out likely, because I doubt that they'd be honest if there was incompetence, and I doubt there will be a proper investigation.