How can a signal backfeed and create a crosstalk in a passive summation circuit?
In an active summer the voltage at the summing node is extremely close to zero but in the passive summer the voltage at the summation point is your output voltage and is non-zero.

This voltage will cause a current to flow though the summation resistors and into the source.  If the source has a non-zero output resistance there will be a voltage generated at the output of the source that is caused by the other inputs.