Why would a lawn irrigation pipe slope down to nothing and just end?
To answer my own question (after a bit of research)...

In terms of following standard sprinkler system layouts, there is no technical reason why a downward sloping pipe would just end and be bent. This seems to be a unique feature to this system. Most likely, the pipe was laid for future expansion. Perhaps budget was an important issue at time of installation, or perhaps the design changed at the last minute. It could be that it was not designed professionally and the layout was changed around a lot through trial and error. All one can infer with the information known is that a pipe is bent and essentially capped off (since water does not seep). It's not causing a loss of pressure in that case. If anything, perhaps the pump is having to work ever so slightly more to maintain the pressure in that zone (since there is a bit of more pipe to cover). If there is little or no pressure in a zone that was at one point working then it has had a change in it's supply, either a loss from leaks, or an issue at its source (the well). But those are whole other issues.