First, of course you only get more than one AoO if you have Combat Reflexes, right? That was not mentioned, although maybe just assumed. (edit: Nail metioned it while I was drafting up this reponse)
Second, Hold the Line gives you an AoO when someone enters your threatened area, giving you an AoO for a charge when you normally would not get one, right?
It sounds to me very much like a movement-caused AoO - basically the same as leaving a threatened square.
Note that:
SRD said:
Combat-AoOs-Combat Reflexes and Additional Attacks of Opportunity
"Moving out of more than one square threatened by the same opponent in the same round doesn’t count as more than one opportunity for that opponent."
does not take into account moving INTO a square because that only provokes an AoO because of a non-core feat, Hold the Line.
Caution is required when using a non-core book with strict core rules text.
You won't find a definitve answer in the RAW here, you'll have to apply some common sense and decide if Hold the Line really creates a new opportunity or just extends the already existing one (moving "out" of a threatened square) to include a new condition (moving "in" to a threatened square).
This sort of common-sense analysis is often required when using non-core feats, spells, abilities etc.