What are the alleged reasons for emergence? What explains emergence, what are the philosophical explanations for emergence? Does it indicate that there's gap in our knowledge, does it indicate that it's something that occurs regardless of whether we know everything or not, is it caused by something else than the lack of knowledge?
Emergence is a general concept and it seems you're concerned with it as a specific philosophical position regarding natural or social phenomena which is termed as Emergentism usually employed in philosophy of mind and related living creatures, self-organising systems, and complex systems:
Other varieties see mind or consciousness as specifically and anomalously requiring emergentist explanation, and therefore constitute a family of positions in the philosophy of mind. Douglas Hofstadter summarises this view as "the soul is more than the sum of its parts". A number of philosophers have offered the argument that qualia constitute the hard problem of consciousness, and resist reductive explanation in a way that all other phenomena do not.
Intermediate positions are possible: for instance, some emergentists hold that emergence is neither universal nor restricted to consciousness, but applies to (for instance) living creatures, or self-organising systems, or complex systems.
As for the root cause of such mysterious emergence, it mainly has three candidates:
- Some philosophers hold that emergent properties causally interact with more fundamental levels, an idea known as downward causation.
- Others maintain that higher-order properties simply supervene over lower levels without direct causal interaction.
- All the cases so far discussed have been synchronic, i.e. the emergent property exists simultaneously with its basis. Yet another variation operates diachronically. Emergentists of this type believe that genuinely novel properties can come into being, without being accountable in terms of the preceding history of the universe.
Of course emergentism strives to be compatible with physicalism and the wiki reference also mentions it should not be considered as anti-reductionsm.
I'm not sure this will answer all your questions, but it might help.
Let's look at water as an example. As a molecule, it does not possess viscosity because that is a collective property of very large numbers of water molecules in close proximity to one another. You can therefore argue that viscosity is an emergent property of water molecules.
Similarly, a single water molecule possesses no surface tension- and neither do a large collection of molecules in close proximity to one another. However, if you furnish a water-air interface, then surface tension becomes manifest. It is emergent, in the sense that the collective behavior of a large number of individual molecules possesses a property that a single molecule does not.
Can a single water molecule experience gravity waves? Certainly not. Can a large collection of molecules in close proximity to one another experience gravity waves? No. If we provide a water-air interface? Still no- but if we add gravity, the answer becomes yes, and we get crashing surf. Same comments apply.
We had a relevant discussion here: Have philosophers speculated on how chaotic forces meeting together can result in order?
I would describe it as a kind of overlay of rules of thumb, where we find structures or units that are in some way persistent. This is fundamentally a kind of symmetry: for instance a cell has a persistence of order across time that the equivalent 'dead' molecules would not. To characterise living matter, we can define it as creating disorder to maintain order, ie 'eating', or harvesting the Gibbs free energy.
Complexity, which emergent behaviour is a subset of, occurs in systems that are between high order, and high disorder. A butterfly flapping it's wings can create a hurricane on the other side of the world because of sensitivity to initial conditions - a frozen world or one on fire would be far less likely to do that, than our warm wet world.
But it's us that makes a conceptual group of phenomena, that sees it as having emergent properties belonging to it, or an identity. It's a shorthand, to support computational traction, where fundamental dynamics can be neglected. For more on that, see: Is the idea of a causal chain physical (or even scientific)?