I'll split this into three different categories as to why I think Pacquiao may have shot himself in the foot for taking this fight.
1) Styles
Pacquiao has always had trouble with people moving. When we observe the first Bradley fight and the Shane Mosley fight, despite winning, Pacquiao showed that he had an extremely difficult time cutting off the ring. Especially in the Mosley fight, we see Pacquiao swinging and missing more often than not. His punch output and combination speed, which has been known as his strongest assets, become null when someone pulls away from him.
Notice how Manny can't ever make a full impact with his punches, aside the few ones that had Mosley on his bike.
2) Age
Pacquiao won the Bradley II fight, but the first 4 rounds were the absolute slowest I've ever seen Pacquiao. Pacquiao began wearing down Bradley and shutting him out after Bradley tried to engage and knock Manny out. When Bradley was pulling back and countering Pacquiao on his misses, Manny was in fits. If Algeiri can properly pull this off, because he's a good boxer-mover and Pacquiao is slowing down, it could be extremely difficult for Pacquiao to close the distance.
Just watch this sparring video to understand what I'm talking about.
3) Corruption
We've seen Pacquiao robbed before by Bradley. Bradley was a likeable guy with an undefeated record. Many speculated that Arum knew Manny was on his way out and wanted to create a new star. Algeiri is a good looking and likeable American who is undefeated. If it is a close fight, I wonder if it would go to the good-old Pacman or go to the newer up-coming guy.
These are all speculation, but I think this'll be one hell of an interesting fight. In my opinion, given what 'could' happen, I think this is one of the most dangerous fights he could take - aside from another Marquez fight. I could be wrong and Pacquiao could wipe the floor with this guy, for all I know.
INb4 Floyd Comments: I think Mayweather is also taking a similarly calculated risk with Maidana. Maidana could be all figured out and get UD'd easily or he could come out even harder and meaner than ever. Both fighters are taking potentially risky fights.