We know Keith is part Galra, which means that one of his parents must be at least part Galra. We've seen Keith's father, and he does look like he is human, and says "your mother will be here soon" in Keith's vision-thing (?) When the Galra soldiers are marching toward their house. This line implies that it is Keiths mother, not his father, that has the Galra blood. It also implies that she is in the Galra army. I believe that she's at least half Galra (which is still enough to qualify as a Galra soldier) and that she was originally for the empire. She might have been high ranking, or at least very smart compared to the others. She spent years tracking down one of the lions to aid her Empire. Her research finally led her to Earth, and the site of the Blue Lion, not far from Keith's father's house. She crash landed for some reason or another. Maybe she went to Earth alone just to see if she was right, maybe she had a few comrades that died in the crash. Keith's dad (KD for now) saw the crashing ship and went to investigate. When he saw Keith's mom (KM) in the wreckage, he decided to pull her out and save her. She was badly injured and couldn't move. He took her to his house, a remote place where she probably wouldn't be tracked, and kept her a secret so the other humans wouldn't panic. KM realized that it was her best chance at survival to play along and receive help from KD, then run when she was ready and alert the galra to the existence of the Blue Lion. The main reason she fought for the Galra was that she believed they were right, and that the Galra were above other beings and would ultimately bring stability to the universe. During her time with KD, she found that she was wrong, and that the Galra were not superior to anyone. She ended up falling for him, and thought they could build a family together. However, when Keith was young, she found a trace of the Galra. Maybe she had scanning equipment that detected the Galra in the far reaches of the solar system. Whatever happened, she knew that she couldn't stay, as the Galra would at some point end up finding the Blue Lion and possibly attacking earth while doing so. To protect her family and the universe, and left in the middle of the night, somehow flying back to space and finding the Empire again, rejoining their ranks and telling them that her research had been false and she could not find the Blue Lion.
You could also take this a step further and say that she never crashed, but was attacked when she reached the ground. The Garrison does try to cover up a lot of things, like we saw in Reunion, on how they knew the Kerberos mission wasn't lost because of equipment failure, but instead something else, and tried to keep the information hidden. Perhaps they are trying to keep the existence of Aliens a secret? Maybe this caused them to attack KM and her crew, and she was the only one to escape.
So yeah thanks for reading all that XD
I'll just go now
*waves arms like Lance did when distracting the Galra soldiers that one time*
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-Your star's still in the sky-
Into the night, out of the dark,
take to the sky, chasing the stars.
All that we said, all that we are,
waiting to fly, this is the start