You don't need to see an agenda behind every invitation to recognize the agenda behind this one.

For those genuinely unclear, it's a combination of things - the individual nature of the invitation, the private nature of the venue, the timing and explicit involvement of libations whose effects often include the lowering of inhibitions - all in all it's just a super sketchy thing to all of a sudden pop up in a previously professional relationship. Any one of those would be fine; any two would be a little questionable but probably okay; all three at once constitute a great big flashing neon sign that spells out "YOUR BOSS IS TRYING TO LAY YOU".