How can unplugging a turntable improve digital?

Yesterday, several guys came up from Houston. My digital sound was quite good with the Exemplar 2900, H-Cat P12A line stage, Reimyo 300b amp, Beauhorns, Indra ics, and the IsoClear plug in box and Super Focus powercords. One of them brought along another Super Focus and asked if we might try it on the Exemplar. I had a Harmonix pc on it replacing the Omega Mikro that I had been using. I had Super Focus cords on the power strip, the amp, and the H-Cat.

After we listened for a while, I did put the IsoClean pc in. It was not broken in and did not sound very good, so we listened to vinyl. Later we retried it and found it much improved but still not the equal of the Harmonix. We then tried the Omega Mikro which was much more neutral.

Then one experienced guy suggested that we remove the Garrard turntable from the IsoClean power bar. I was beside the speaker doing this and immediately heard the improvement. Later we put the IsoClean back in with the table still not connected. It too was greatly improved.

Last evening I put the Harmonix in also with the turntable still not plugged in. I really thought it too was much improved and listened again to many of my disks, finding all sounded noticably improved.

I have always thought the "electrons don't know what kind of wire they went through" guys were nuts, but I really don't understand how what is at work here. I am certain, however, that I will never go back. I have found means to plug the table into another independent outlet.
I think what is happening by plugging turntable into another outlet you removed the digital "grunge" put out by your CDP on the AC line. Note: this effect is noticable even when CDP is off. Effect is similar to a light with a varisitor for on/off control which are NOT recommened for same circuit audio is on.
Perhaps your turntable has a "dirty" power supply and you are experiencing a ground loop with the TT plugged in? If that's the case then the loop would be broken once you unplug the offending component from the AC mains, which would at least be consistent with what you described.
Oxia, by dirty do you mean leaking to ground?
Yes, Tbg. From your final sentence ("I have found means to plug the table into another independent outlet"), I take it that you have found an effective workaround?
Walker Audio has stated that their product, the Precision Motor Drive, isolates the AC Turntable motor from your digital system and improves digital playback.

Walker's Web site says:

The Precision Isolated Power Motor Drive also prevents electrical noise from the AC drive motor from contaminating the AC power supply for your other components. With the Precision Isolated Power Motor Drive in your analog system, you will immediately notice a larger, stable, transparent sound stage, with faster dynamics and better harmonics, without grain or glare.

So it appears that Walker agrees with your findings.

I have no expertise on this but I have been told by friends that the main reasons for this phenomena are twofold:

1. RFI - the connecting wires and power cord act as an antenna, putting out and receiving radio frequencies which color or cause artifacts.

2. There is a "back wave" where the wire meets any resistance, which interferes with the signal.

These are supposedly true for AC lines and high or low signal carrying lines. Using these premises, I have found that anytime I remove extra lines, cover lines with RFI damping material (believe it or not, cotton is incredible in its properties for this) or separate the lines, I get greatly improved and cleaner sound.
Tedvan, I once had the Walker tables and entirely agree that Lloyd's power supply does protect the ac mains from the motor noise. But the Garrard 501 has a regenerator also that should act similarly. There is something about having the turntable electronics, even while off, tied to the digital electronics which dulls the detail and dynamics of the digital sound. I guess it could be that the ac is seeing something on the turntable line that is influencing the sign wave or the impedance of the ac source.

Oxia, my work around was plugging the turntable into another ac circuit. Actually, even plugging it into the same circuit but through a Sound Application Line Stage filter also avoided the problem.