Your opponent let you play a greek gift sacrifice on them. It's quite well-known and when black is castled kingside, if white places a bishop on d3 with no blocking by a pawn, it could be quite dangerous at times. The sacrifices are hard to spot too, which make it even more dangerous when spotted.
@SehathAbayasinghe Qb8 was a passive move by Black. And then b5 and a5 was adding trouble to Black's woes and it was made lost by Black itself. You were actually lucky to win.