The fires apparently take place after numerous failed attempts to start a vehicle. A current limiter in the starter motor can overheat and melt, thereby causing a fire. According to Mercedes, this issue only prevents itself on vehicles that had previously had engine or transmission damage serious enough that the starter motor couldn't turn the engine over.
Affected models include the CLA-, C-, and E-Class sedans, and GLA- and GLC-Class crossovers. Click here for a PDF with all the details from Mercedes. If you own one of the affected vehicles, expect to be notified once in late March, and again when parts become available in July.