It is mostly a problem of DRAM memory cells, though theoretically with enough energy it could flip SRAM cells to or override the driver of a given wire. It is not specific to voting machines.

But the main source is from cosmic rays and local radiation sources in the ceramic packaging and/or decaying elements in the metal frame/leads/solder.

Other types of particles interact much more frequently. But yes: bits do get flipped (even with ECC memory).