Semiconductors are subject to electron migration- in normal operation, silicon is doped to be either P (missing some electrons) or N (extra electrons). It can eventually break down the doping enough such that a semiconductor no longer "switches".

Similarly, unshielded gamma radiation from the sun and space in general is pretty rough on solid state electronics. It is ionizing radiation so it can affect some semiconductors over time as well as actual mechanical changes in some metals.

Also consider that the Voyager probes rely on a RTG (Radioisotope Thermoelectric Generator) for their electrical power, a device that uses nuclear decay heat to generate power. These sources also slowly lose output over time as the radioisotope is consumed and the thermoelectric couplers break down due to the radiation exposure. When Vpyager 1 launched, it produced 470 watts of electrical power for its sensor and computer systems- around 1998 they started turning systems off, and by now it's down to around 200 watts, which is not much considering the power demands of the communication dish. When supply voltage starts to drop, previously OK electronics can begin to show errors where the defects exist.