Aircraft are designed so that a lightning strike is not catastrophic, but it would be prohibitively expensive to arrange that they are completely unaffected. When the strike current flows through the airframe it typically makes some interesting voltage surges in nearby wiring, and any directly attached bit of kit may be affected. It is not unusual for bits of electronics to be confused or damaged as a result. Since this incident seems to have resulted in the cabin climbing such that the crew decided to descend, it sounds like something in the pressurisation controller (or the data that drives it) got zapped.