Caused by the hot air solder levelling process when a manufacturer skips a process or has the process order slightly out. The solder should only be deposited on the component pads, but some manufacturers used to put the solder on before the silk screen, then level the board. The solder melts during levelling and aggregates under the mask forming the "wrinkles". It should be - silk screen, apply tin/lead to pads, level. It can be - tin/lead, silk screen, mask, level. but if its - apply tin/lead, silk screen, level - you get wrinkles. Wrinkles became less prevalent as tin/lead material costs went up, does not happen now as tin/lead here (UK) is banned except for special applications.