@Olias has asked some questions that may help us to make better guesses.
In the meantime, if my memory serves me right, the X1 uses a sensor in the build platform to detect when the nozzle makes contact, rather than a sensor in the print head itself. With that in mind, you might want to check for any debris or obstruction near the nozzle that could be contacting the build plate before the nozzle tip itself.
While this is somewhat unlikely given the efforts the printer makes to remove such obstructions during pre-homing operations, it is not impossible. Since the “vibration sensor” is in the bed, anything rigid enough to “tap” the bed could cause a false homing trigger. It wouldn’t even have to be part of the print head assembly, although this scenario is quite improbable.
If all else has failed and you are desperate enough, you might consider looking under the printer. The drive belt for the Z-axis lead screw is exposed down there. There would likely be other signs of trouble long before the bed got within 4mm of the nozzle, but it’s worth checking just in case.