For my first post I will talk about the resons that at least some ufo crashes might be real. There are two main things that could cause a ufo to crash on earth. 1. The ufo was shot down by another ufo that was attacking it in some kind of war. 2. The ufo was some how shot down by people. Using space based weapondry or some other weapon system.
You forgot this reason: 4 the saucers could have malfunction and this one 5 the drivers of the saucer could have mis-calcutated and slamed into the earth and this one 6... and this one 7..... ect
The things I listed where assuming some ufos are real and assuming that for them to get to planet earth then they would have technology sophisticated enought not to spash into the planet by mistake. For another theory maybe they originated on earth but not nesseraly from humans. Maybe another civilization thats been on the planet and evolved before us and continues to be seen.
