Box selection in orthographic view ports with nothing previously selected will select BSP brushes even when you are not passing over edges or vertices. The current expected behavior is that it would only select brushes when their edges or vertices are in the selection box. Currently we're seeing it select BSP brushes simply by passing over the faces of the brush (which aren't even visible to the user)
1. Have a map with multiple brushes that are overlapping
2. Attempt to box select one brush without selecting edges or verts
All brushes beneath that selection box will be selected
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-9866 in the post.