It is likely from the preflash of one or more Nikon flashes. There is a wide variety of RadioPopper remotes, so I cannot be specific, but if they are TTL compatibel remotes, then there is a preflash that helps meter how much light from the flash is required, which happens before the exposure flash, and that will trigger Einstein early.
Even if the flashes are set manually, some remotes rely on IR or white light "Morse code" signals, even some Radio Poppers, which can take the light signals, translate them to radio, then back. This "Morse code" of optical communication also happens before the exposure, and would still trip the slave eye.
This is not a problem unique to Einstein, as any basic optical slave would act the same way.
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