Answer: We brainstormed here and came up with a few possible causes. First, peas need quite a lot of water to swell and sprout, so it could be that the half that didn't germinate didn't get enough water. What sometimes happens is that seeds start sprouting in moist soil, then the soil dries out and the newly germinated seeds die. They need a consistant supply of moisture during their first few weeks of life, until their roots get established. Perhaps some seeds sprouted within a week of planting, but then dried out before they got the rain a few weeks later. If a seed has sprouted, then died, it would be difficult to find by sifting through the soil--it would decompose fairly quickly.
Another possibility is that the new sprouts were pulled up by birds. They may have done this early in the morning, before you were out to watch. Other creatures--mice, etc.--might have dug them up, though you probably would have seen signs of that. In cold, damp soil peas can rot before they get a chance to sprout--but that doesn't sound like the case in your garden. Finally, some varieties of peas seem to have better germination rates than others.
This is a good example of why it's important to sow your seeds thickly, then once they get established you thin them down to the proper spacing. Life is perilous for a tiny seedling!
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