My Mac spent weeks uploading 100,000 photos to Lightroom Mobile. Now, after upgrade to Yosemite, it suddenly shows "Syncing 100,000 photos" in status. Why does it think it needs to re-upload? Would it happen on every OS X upgrade? This is a very serious regress...
UPDATE: Actually it's going through them very quickly, seems to be some checksum comparison rather than full reupload. Already at 66K. Still wonder why OS upgrade triggered it.
Could it be that you made metadata changes or a 'Save Metadata to file' on all images, that would trigger such an update.
An update to Lr itself is very likely to cause a resync of metadata as we often add metadata fields to sync. So if you did that, you'll see it go to 100000 in your case and then quickly drop down.