Sparking when connecting mains powered USB device

[Zbig]

Just a while ago I have freaked out a bit when I saw (and heard) sparking/arcing between the cable's USB A plug's shield and my laptop dock's socket when connecting my active monitors. My exact setup:

• A Dell laptop connected to a TB16 Thunderbolt dock (for power, networking and all the peripherals)
• TB16 dock is powered (and is also powering the laptop) from mains via a power supply that's using CEE 7/7 plug, i.e. earthed.
• Active monitors are Yamaha NX-N500. They use C18 sockets for power, so I believe they are floating. Other than being connected with a laptop's docking station via USB (where I'm getting the arcing), they are also connected to the Ethernet. AFAIK, every Ethernet-connected device is galvanically separated by design so no path to ground there, either, I think?
  
• There's also an external LCD monitor connected to the docking station via Display Port.
  

I have measured the AC voltage between the shield of the USB cable connected to the dock and the shield of the speaker's USB port and got ~110V. I live in a 240V country so I think it's to expected for a device to float at somewhere around half of the mains voltage, right? On one hand, I know it's designed this way and that's why the shields of the USB connectors mate first and all that but, on the other hand, I'm still kind of freaked out a tiny bit by seeing actual sparks coming from my (expensive as hell to me) setup. Shouldn't the speakers' USB port be isolated? Or should I get a life and calm down?

[Jeroen3]

On first connection there is some equalization current. If the initial voltage is high enough it might spark.
It doesn’t have to be leakage from floating supply, might also be ESD.

This is why connectors always connect chassis or negative first.
Audio plugs do not, and this is what you can kill your equipment.

[LapTop006]

On the chance it's not leakage from the Yamaha monitors it might be worth getting a tester and ensure your earth is actually connected and not floating.

Yamaha are the sort of company I wouldn't have expected to stuff up a simple power supply.

Non-PoE ethernet should be isolated to 1.5kV.

[Berni]

It could be static discharge since laptops often end up floating from ground. I even managed to bluescreen a laptop before by plugging in a grounded charger plug while the laptop appeared to be charged up to a significant voltage.

Other thing is that switchmode supplies often have EMI suppression capacitors between the mains and the output, these can charge up to half the mains voltage and dump the energy when given ground all of a sudden. Its mostly a PSU design choice. For example i found that a Apple Mac Mini has some pretty big caps in there to mains because i could feel the 50Hz mains when touching the case. For fun i grabbed a blue LED and held it by one of its pins and then touching the other pin to the metal case and the LED lit up brightly enough to be easily seen in daylight. (And btw that Mac Mini had a 2 pin mains connector on the back so you couldn't use a grounded mains lead even if you wanted to)

EDIT: Btw the floating case problem was easily solved by simply plugging it into a computer monitor since those have proper 3 pin IEC connectors so they are grounded, connecting a DVI cable to the Mac Mini then also grounds it via that cable.