The Death Penalty Information Center, which is to release the report tomorrow, attributes the decline largely to growing public awareness of death-row exonerations and concerns that innocent people might be sentenced to die.
In the Talk Left post there doesn't appear to be any discussion of a couple other factors that could cause fewer death penalty sentences. First, it could be because prosecutors are simply not going forward with as many death penalty cases. Second, it could be that there are other reasons the jury decides not to sentence death.
The second issue is pretty difficult to determine. I'm sure that the public has become more aware of exonerations, DNA tests, and other death penalty related news that has increased the publics awareness of problems with the system. However, that doesn't mean they won't issue the death penalty if they are trying a case where they truly believe the person is guilty and deserves the maximum penalty. What seems more likely to be the case would be that the facts of the case aren't strong enough or there are mitigating circumstances.
I'm interested to see if this report addresses other reasons the death penalty might be issued less frequently. Perhaps I'll make a little time to ponder over the report.