Obstruction?

[SC Ump]

Not sure I understand the scenario properly but...

Sounds to me like either (a) the fielder was first making a play on the ball, and then once it was fielded, went into a mode of making vain attempts at trying to tag the runner, or (b) the fielder was fielding the ball and as completing that task accidently stepped on the batter runner's heel.

In either case, I do not see obstruction.

If however, the fielder intentionally tripped the runner as in your "I can imagine" statement, I would have USC. If the fielder was flailing about in a haphazard attempt to rollover or sit upright, I don't think I would have a call.

Either way, I bet your going have a coach griping.