I'd say there a lot of reasons for lulls, and I'm not sure there's anything like "THE LULL(S)" . Let me give you some examples.
1) Just last weekend, I hit a minor lull in activity that pretty well matches other lulls I've encoutered on my property in previous years. The deer just disappeared, and it looked like they hadn't been there all week. My guess is that there's something tasty that comes ripe just at this time and the deer all flock to wherever it is to eat it. In past years, I've stayed home the 4th weekend just because there's so little to hunt, but the last couple of years my sons have become interested in Fall turkey. I know the deer will be back-- either this coming weekend or the next.
2) I have another lull in activity in September. My deer appear to be taking the month off at the shore. There's nothing to see the whole month. This year, it rained all summer, and for some reason that made them decide to stay.
3) I once hunted a big dairy farm over by Warsaw that had the biggest lull I ever saw: massive sign up until bow season started and then nothing. The deer didn't come back until December. My buddy that owned the place tried everything, and finally sold the place. We found out two things:
a) There was a group of guys that were coming in and poaching the place just before season.
b) The neighbors were reporting influxes of lots of deer right after season started.
My point is that these lulls may be very localized, very situational, and somewhat fickle. They can't be universal, because the deer HAVE to go somewhere. When my kids were little, I used to tell them that the deer took off and went to the beach, but in reality I surmised they'd probably left the ridge tops where we are, and went down into the river bottom a couple of miles away to eat crops. Somebody down there was wondering where all the deer were coming from.