There was a good analysis of this elsewhere that effectively boiled down to how the tech being used (I want to say LIDAR, but it could have been lasers or cameras) had evolved. It's apparently really good at detecting how fast other objects are going, and really bad (or, at least, not designed well in its current state) at detecting stationary objects, since it's primarily been trained to ignore the millions of stationary objects all around it throughout a drive to be able to focus on moving objects (cars, pedestrians, bicyclists, etc).

Apparently, moving and stationary objects are treated differently, and both have to be intentionally trained, and that this was a case of seeing the stationary object but ignoring it, just like it does with most other stationary objects that aren't in the road. I'm not technical enough to go into specifics, but that's the gist from the article I saw-but-can-no-longer-find.

You're mistaken, that was about radar not LIDAR. Telsa cars have no LIDAR.

The radars in use have poor angular resolution meaning they can tell if something is moving toward or away from it, but cannot tell where that object is. Since driving down the road will necessarily involve many objects on the side of the road, but the radar apparently can't tell where they are because of it's poor angular resolution, the 'solution' was apparently to filter out everything moving towards the sensor at the car's current speed (e.g, all stationary objects.)

It's unclear to me what if any circumstances will cause the emergency breaking system in a Tesla to fire.