I've had to do this ~3 times in the last couple years. In all cases it was extensions not playing well (xcache, gmagick/imagick). I actually couldn't find a single version of gmagick/imagick that tore down or something properly in php 7.0.x (broke when phpunit launched extra-process tests, among other places)

I've always found the cause to be extensions as well. Usually a version conflict or something. It's so difficult to debug though. Always takes a while just to get everything you need to finally dig in.