Click Here to reference the Mechanical and Hydraulic Operation of these lifters in the Sportsterpedia.

The quick answer is it just takes a little time for system oil pressure to make it inside the lifters and pump them up.
This is quite normal on fresh builds and does tend to scare one that isn't looking for it.
The act of installing the rocker boxes will bleed each lifter down to a low oil state.
So even if you do have the lifter fully pumped before installing it, it won't be that way when you fire up the engine.
In fact, they will all be low of oil and most likely will bang at such a rate to sound like they are about to come out of there.

When installing lifters in the motor, it's best to make sure each one is fully pumped up (full of oil and plungers holding tight, not spongy).
This will ensure each lifter WILL pump up and is not faulty (bad thing to learn on a too late notice).
That said, you may hear from many folks on the best way to oil the lifters before installing them.
However, you can't pour oil down these pushrod tubes to stop it since the oil that is needed is behind a check valve in the bottom of each lifter.
All that would do is fill the pushrods thus the oil reservoir above the check valve (if oil can make it past the restricted disc under the pushrods.
Some say to soak them in a container of oil overnight.
Others will dunk each lifter in a container of oil working the plunger up and down until it doesn't move down at all (faster and more accurate anyway).
However, it's done, the important thing is to lube to rollers of the wheels good and insure there is at least (some) oil inside the lifter body.

The butt squeezing “did I just ruin my motor” feeling is normal as long as the clacking noise soon goes away.
Once oil pressure finds it's way into the lower pressure chamber in the lifter (not just to the lifter feed hole), lifters will pump up and the noise goes away.
It shouldn't take too long at all for oil to arrive and pump the lifters up.
It would depend on how thick oil viscosity is and how long it takes the oil pump to get the system pressurized.
Oil flows slow when not at operating temp, slower when at room temp, slower in cold weather etc.
So how long the lifters will bark will most likely be different on each bike / situation.
Expect seconds to a minute or two and don't wind it up until the lifters quiet down.
If they are noisy, cracking the throttle won't get oil to the lifters as fast as the lifters rattle around.

The animation below runs from lifter installation to firing up the engine showing the different stages the pressure chamber goes through during a build.
The pressure chamber is the lower compartment under the check valve and is responsible for actually lifting the engine valves.
The inner plunger oil reservoir is above the check valve and most likely will not run out of oil.
But the plunger will hit it's hard seat anyways when the oil level in the lower chamber gets too low.

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