It is happening because it's bugged, and there is not much you can do about it, other than wait for it to be fixed.
The "Nearby Pokemon" section has been known to be buggy from the initial release. In fact, there are reports that the problem got a lot worst in the last 24 hours, after a brief period where the mechanic did not function at all.
It is speculated that the issue has something to do with the Google APIs, supported by the fact that the location map showing where you caught each individual Pokemon has gone completely white for most players.
It is assumed that Niantic are aware of the bug, and are working on a fix. Regardless, it is recommended that you submit your own bug report, as more reports could increase the priority of the bug fix over other issues Niantic are currently trying to solve.