PLX-1000 Ground Issue?

I bought a pair of PLX-1000 decks 2 weeks ago and have only used them with the DJM S9 mixer. Out of habit I've been grounding them even though I've only used them with Serato control records. Last night I tried hooking them up without grounding them since I've been told you shouldn't have to (with serato control records). Well, my right deck worked fine but sure enough I got a large amount of interference on the left side. Then I hooked up the ground wire and everything seemed to be tracking well, nice and clear circles on the scope views in serato.

So what could cause this? I use an S9 mixer. I tried swapping needles. I tried a different set of RCAs. I tried plugging the turntable into CD/LINE instead of phono. I made sure Serato DJ settings were set to "Turntables" and the S9 utility was on  CONTROL TONE PHONO for channels 1 and 2. I tried turning off the mixer and back on as well as the turntable itself. I think I went through just about every troubleshoot you can think of.

Thoughts anyone?

Here's the scope view of the left deck not grounded.