Whether or not the contact is intentional has no bearing on the outcome of this call. Your question is really "is clipping from behind considered as a foul?", and the answer is yes.
From the 11th Edition rules:
Foul: Non-Incidental contact: contact between opposing players (see
II.H for a definition of incidental contact). In general, the player
initiating the contact has committed the foul.
Incidental-contact: Contact between opposing players that does not
affect continued play.
The situation you describe is definitely non-incidental, so is a foul, and it is a foul on the initiating player. The trailing player is almost always the player initiating contact. It's not "out of nowhere" that their back foot clips your thigh. They were running. You were running behind them. You were in complete control of how close you were to them.
This is supported by USA Ultimate's Standing Rules Committee in this article:
The answer is that it's a foul on whoever initiated the contact
(II.E). In practice, this is often hard to determine, but usually
means that it’s a foul on the trailing person, since the motion of the
person in front is directed away from the other player, and therefore
that player is less likely to have been the initiator of the contact.
If it's really impossible to say who initiated the contact though,
then it should be treated as a case of offsetting fouls, and the disc
should go back to the thrower.
For the situation where you are standing still, and another player twists, runs and clips you, then clearly you have not initiated the contact and therefore have not committed a foul.
Here's another quote from USA Ultimate's FAQ supporting the downfield player (even if you are impeded). This quote talks about a defender moving into the path of an offensive player to box-out, but it equally applies to an offensive player moving into the path of a defender to box-out.
However, if a defender is playing the disc (e.g. looking at and
reacting to the trajectory of the disc in order to make the catch/D),
they are allowed to move into unoccupied space for the purpose of
preventing their opponent from taking that space (ie- “boxing out”),
since they are not solely playing their opponent.