If you plug while the power is on, it may be overcurrent yes.
Even already plugged, it may be too much depending on how bright the setting is.
My PC has all the fans connected to one channel (81 LEDs), and i can shut down the channel when setting it to white and half way in the brightness. Aquasuite diusplays a warning when it's connected telling you the channel was shut down to protect from excessive current draw.
That's what may be happening but you don't see it since they are not plugged to USB.
Maybe connect them and set a dimmer profile for your tests?
And preferably plug the fans with the power off