There's apparently a duplication of ground paths (ground loop.) When you have the ground wire tied to your TT/arm (which is the norm) and the ground pins of the cartridge are also tied to your TT or tonearm (not normal) that equals two ground paths to the preamp.
That is not a typical situation but it does happen with some kinds of TT/tonearms. Normally the "ground" pins of the cartridge are simply connected to the preamp thru the ICs and nothing else. The cartridge remains otherwise isolated (cartridge ground pins are not usually connected to the cartridge body either.)
The TT/arm are separately connected to the preamp ground lug thru the ground wire (but not to the cartridge or the L and R RCA leads in any way)
So disconnecting the ground wire eliminated (in your situation) the duplicate path and thus the loop and the hum.
The reason the hum diminished when you touched the ground lug (with the wire still connected) was that your body acted as a big ground drain on that leg of the loop (the ground wire) and thus reduced the effects of the ground loop somewhat, but not entirely as you noticed.
How, where or why the ground pins of your cartridge (or possibly the shield of your L and R RCA cables at the TT end) are also connected to the arm/TT, I don't know. You'd get better sonics if they were not, and you used the ground wire instead; because right now, your cartridge signal leads (in the RCA cables) are also carrying the ground drain to the preamp, and it would be best if they weren't.
That is not a typical situation but it does happen with some kinds of TT/tonearms. Normally the "ground" pins of the cartridge are simply connected to the preamp thru the ICs and nothing else. The cartridge remains otherwise isolated (cartridge ground pins are not usually connected to the cartridge body either.)
The TT/arm are separately connected to the preamp ground lug thru the ground wire (but not to the cartridge or the L and R RCA leads in any way)
So disconnecting the ground wire eliminated (in your situation) the duplicate path and thus the loop and the hum.
The reason the hum diminished when you touched the ground lug (with the wire still connected) was that your body acted as a big ground drain on that leg of the loop (the ground wire) and thus reduced the effects of the ground loop somewhat, but not entirely as you noticed.
How, where or why the ground pins of your cartridge (or possibly the shield of your L and R RCA cables at the TT end) are also connected to the arm/TT, I don't know. You'd get better sonics if they were not, and you used the ground wire instead; because right now, your cartridge signal leads (in the RCA cables) are also carrying the ground drain to the preamp, and it would be best if they weren't.