They're programmed, but I have seen huge hatches and nary a fish feeding. And I've seen them go crazy over a few bugs.
Just be thankful, as I've fished Penn's a lot, and the typical situation at this time of year is that you get about a half hour of good fishing right at the end of the day. You maybe pick up a fish or two nymphing while waiting for stuff to happen. Sulphur duns will start at like 8 p.m and not get much response until sunset. Just when you think about leaving cause it ain't gonna happen today is when you see a rise, then another, and it gradually works up to a crescendo right as it becomes too dark to see, and you're tossing flies to the general vicinity of rises you heard, not saw to try and save the day, while bugs are smacking you in the face. You're setting hooks on sound of rises, and finally, there's resistance, only to have it snap off. Son of a.... 6 fish is a good day.
But, yeah, it can sustain goodness for some time on rare occasions and this year seems to have had more than it's share of them. Low clearish water, low cloud cover/no sun, and early day hatches is a magic combination. You need all 3, and that's a very rare combination. Yes, I've seen it. I don't think I ever topped 50 in a day on Penn's, but I have been around 30ish. It's very rare. I've been skunked more than a few times too.
From the sounds of it, rain clouded up the water then the sun came out. Yup, that'll shut Penn's down, regardless of what the bugs do. Sun and Brown trout don't mix.