Another dropped infield fly sitch.

[SethPDX]

I think the first two responses were very good explanations. I would just add:

Originally Posted by referee99

Batter hits towering pop up to F6 who is playing deep on the turf field.
I may have taken a shade longer than normal to call (making sure it was IF), but yell "Infield Fly!" (I have a loud voice) soon after it reaches its apex.

Taking an extra moment before calling the IF was a good move on your part. Some umpires think this has to be done ASAP. There is no need to call an IF immediately; make sure it meets the requirements, then call it, like you did.