Why Power factor changes from primary side to secondary side of the transformer

I have a query about the power factor change in a power plant. There are 8 gas engines in parallel. When 1 engine runs with a set p.f of .8 the output at the delivery after transformer (11/132kv, step up Ynd1) is almost same. Power is supplied to the grid having 132Kv. But when all generators are running with each having .80 p.f the resultant p.f after transformer changes to .85. (All gensets are in parallel and connected to single transformer and distance from gen to trans is comparatively small). Why the p.f increases please explain elaborately.