The spell Forceful Strike allows you to make a free Bull Rush attempt when you cast it. But besides the normal spellcasting AoO (which can be mitigated by casting defensively,) wouldn't this spell also provoke a second time unless you have Improved Bull Rush?
edit: as pointed out, since it's a swift action spell the casting doesn't provoke. But, it still seems like the bull rush provokes, which seems odd.
RAI it's probably not supposed to provoke, but RAW the clause that says it doesn't (like for example in Hydraulic push) is missing.
Just as a note quickened/swift action spells do not provoke so it does not need the language to not provoke because it is a swift action to cast.
I had forgotten swift action spells don't provoke, but the Bull Rush attempt should still provoke.
This is somewhat of a moot point since you can 5 foot away after full attacking and cast if you don't have imp bull rush and/or greater bull rush but I think GBR would make the target provoke.
Forceful Strike doesn't allow you to 5-foot step away before casting, because "You cast this spell as you strike a creature with a melee weapon".
It does not provoke. This is because you are not performing a combat maneuver:
The force of the blow may be enough to knock the target backward as well. To determine if the target is pushed back, make a combat maneuver check with a bonus equal to your caster level to resolve a bull rush attempt against the creature struck.
You are not bull rushing. Instead you are running the check which determines if the spell pushes them back. Combat maneuver checks do not provoke (combat maneuvers do in several cases but a check does not). The check isn't the maneuver in this case, it's simply used to determine if the creature hit is moved.
It is the Bull Rush Action that provokes an AoO. The spell does not, as the OP writes, "allow you to make a free bull rush attempt". It is a spell that augments a melee strike, and uses the bull rush mechanics to determine its effect. There is a reason it doesn't use your CMB for the check, but instead uses caster level. It is the spell doing the pushing, not you.
I agree. You are not performing a bull rush maneuver, so you are not doing anything that would provoke.