Expanding trees ..?

Hi,

There's something odd happening to some of the trees on my terrain. When I move the first-person controller in across the terrain, some trees expand in a most unusual (and slightly creepy) manner when I approach them; and they shrink again to their normal proportions when I back up again. It doesn't happen with all trees, but with about one in every four.
I made a screencast of the problem viewable here.

Some details: running Unity 5.1.0.f3 personal on mac osx 10.10.3. Terrain consists of a 4x4 grid of 2048 x 2048 size tiles. Terrain is built with external height map plus Terrain Composer. Using RTP v 3 (current version).
The trees are the free broadleaf_desktop and conifer_desktop speedtree samples from the asset store. The ones that show the described behaviour are all of the speedtree broadloeaf_desktop type.

Does anyone know what might cause this behaviour? It's certainly unusual, and it reminds me of a strange SF story I once read (though I forgot what it was). But I'd like to lose it all the same

Thanks in advance,
Lúthien

 

It's been reported already to Unity, according to a forum post, the first patch for 5.1 will fix those issues. Otherwise, drop back down to 5.0.x and that issue goes away.