I'm pretty sure the attack on Fort Irwin was one of opportunity. The chance that pinstripes or randy would have figured a recon team would come in, plan to get an operative on the helo outbid town, turn some helpers into Zeds, and attack the base seems too remote.

Instead, the maker of the little ones probably set them up to be on the lookout for humans and decide some how or another when to kill and when to turn them. The LO's have some way to determine when to carry out an attack vs when to wait. The one on the helo out of LA waited a while before making its move and taking them down. And some number of the turned in Boulder held out long enough to get closer to Fort Irwin.

For the ones that weren't made directly by "maker", I wonder what drives them and how much decision capabilities they have...

Like Burt said, "I'm a pretty good shot when they're just running at you mindlessly. But..."