I think Dr Eric Davis was referring to paranormal activity within the confines of the Skinwalker Ranch.
"went quiet for awhile" = for an extended period of time, a sharp reduction in reported paranormal activity within the ranch boarders.
"heated up again" = a recent marked increase in reported paranormal incidents.
From what I have read, "phantasms/entities" entering or exiting "portals" are a few of the things reported.