What's Killing My Plants?

Last year, I planted a couple of mugo dwarf pines on either side of the front steps coming up from the sidewalk. But by summer's end, I'd lost one of them to some slow, brown death. I replaced it this past spring.

But now the other one looks like it's dying:

I'm also sad to see that, of my four ice plants, I've completely lost one, a second is well on its way to the graveyard, and a third one is beginning to be affected:

Here's where my landscaping ignorance is busting my chops.