Diagnosing connected circuits; dangerous?
Because you are doing something you should not be doing: You are interrupting the neutral wire for the other half of the MWBC.
A Multi-wire branch circuit (MWBC) is 2 hot wires on opposite phases that share a neutral. As long as they are on opposite phases, the neutral wire carries only differential current.
On a MWBC, you are not allowed to route the neutral through a device.  So for instance you should have found neutral pigtailed to the existing receptacle.  You may have thought &quot;well that's silly, GFCIs can take 2 wires on their LINE screws using screw-and-clamp... why don't we just eliminate that completely irrelvant pigtail?&quot;  ZAP... that's why.
Neutral must remain intact even if a device is removed, e.g. if you need to change a receptacle.
Note that if an MWBC's shared neutral continues onward from any particular box, you will not be using the &quot;Load&quot; terminals on any GFCI receptacle you install there.  If your GFCI is monitoring L1-neutral to assure flows are equal, then any current flow on L2-neutral will cause neutral to be unequal.
There are only 2 ways to GFCI-protect a MWBC:

A 2-pole (3-wire) GFCI, which are only made as GFCI breakers.
Individual GFCI recep at every outlet where neutral is shared. &quot;Load&quot; is not used.