How often do you see this

You are betting down with middle pair against a LPP and on the river when a nothing card falls they bet into with a draw that they missed. I'm not sure exactly what is going through their head.

[Harv72b]

They are trying to mix up their play and get you to fold naked overcards or another missed draw. Note that if a player is doing this against you, they are probably not really a loose/passive.

Especially if you raised preflop and the board came all low cards, this is a line that many players will take with a pocket pair...so it only makes sense that every so often they'll try it without a pocket pair.