over the horizon radar is always very low frequency stuff as reflection on various layers is used to get the signal over the horizon. those layers are largely transparent at 1090/1035 MHz.
I’d go with intentional jamming/RFI causing at ModeS frequencies. That’ll confuse the XPDR system.
edit: if it’s only GPS jamming or spoofing (which is equally easy to do) you’d get various IRS nav disagree messages I’d presume.