Stuff gets old. Connectors corrode and become intermittent. Fuse blocks stop making contact. Switch contacts go bad.
Also, repeated thermal cycling makes solder joints fail, especially if they weren't well-made. And electrolytic capacitors fail, although all things considered, BMW has a lot fewer electrolytic failures than just about anyone else. They seemed to use very reliable caps.
BMW has a tendency to add outrageous amounts of electrical stuff into cars, and consequently they suffer an outrageous amount of electrical failures. The more stuff you have, the more stuff there is to break.
I recommend pre-emptively packing all electrical connectors with dielectric grease, which will prevent a lot of the issues. And when individual modules fail, whether that be the ECU or the turn-signal flasher, first look for cold solder joints before doing anything else.
--scott