I'll be curious to hear other responses. The top hand is a big part of the equation. In order to maximize bat speed and power, it must be fully engaged through the contact zone. For most, a good gage to measure that is where the top hand ends up at the finish of the swing. If it ends up at least up near the shoulder of the bottom hand, then top hand release was not too early. If it ends up low and across the mid-section, this is often an indication that release was early and the hitter didn't stay connected long enough.
Beyond that, it comes down to preference, swing type, flexibility and second half extension. Some keep both hands on, some release the top hand. Also, it is often a result of pitch location.
Often, when I am working with some of our hitters in the cage, I will ask that they keep both hands on the bat. This may be because I am seeing early release or because I am seeing the swing get long too early or pulling off the ball early or it is a balance issue. Once the issue is resolved, I am OK with them going back to top hand release, as long as it isn't early and doesn't cause re-introduction of the initially perceived problem.
I agree with the variables as to why the top hand could come off. I want to keep 2 hands on the bat, but the one handed follow through just happens.