If somebody could answer that question, and be right about it, they would be worth a bunch of money..........mostly from me!!!!! You have several things that are very different about your lakes out there, so pinpointing a reason will be difficult. Without more info, it will be hard to speculate. But, if I can ask some specific questions, maybe we can help. 'Ole slowretrive is an expert and tracking down those disappearing hybs.
One thing that stands out to me is you indicated that the water temps (presumably surface) are just now in the lower 70's. Our lakes are mid 80's right now, and have been above 70 for three months.. I can not see a distinctive thermocline setting up with those low temps in your lake. We just have begun to see signs of it here. Can you see signs of the 'cline on your finder?
Here, when temps get to 65-68 degrees, our stripers, (and hybs) go through their pseudo spawn, in which they drop their eggs. That would make sense since the timing of their (disappearance) seems to coincide with those water temps. That is a two to three week period, but not all go through it at the same time.
Hybrids are a lot more heat tolerant than their full blooded striped cousins. 70 degree surface temps is kind of the "magic" number around here for them to be all over the water column. In the summer, when the 'cline' puts the squeeze on them, they will choose the coolest water temps with the most oxygen to spend their time in. They will also choose cooler water over more oxygenated water to spend time in.
So let me ask some questions:
What is the primary forage (bait) in the lake?
What is the hottest the surface temps get in the summer?
When is iceout for your lake?
What is the deepest water in your lake?
What is the biggest your hybs get?
When are you going to take me fishing so I REALLY give you a good answer?
I know, a bunch of jibberish, and I still didn't answer your question.