Taitz states their view in the video: it's because his mama (herself an American citizen) was in Kenya for some reason and had her baby and didn't want to deal with the hassle of immigration. So naturally she raced back to the states, got the newspapers to forge the birth announcements then produced a fake birth certificate to seal the deal. All this in an elaborate scheme to one day destroy America.
It's really a vast conspiracy to them that involves a woman who was clearly a sketchy character! She did have a child with a black man, and that should tell you something about her character right there! So naturally she'd be in Kenya doing Kenyan things like shitting on the American flag and cursing the good name of John Kennedy and plotting how best to overthrow this opulent country. The best way to do that was to fake the citizenship of her son who was black, not from a political family, and who at the time would have been viewed as having no shot at the presidency. These America-haters are crafty type.
This has two layers though. One, that Obama is not a citizen and shouldn't be president. This is easily debunked, and really comes across as racist garbage. Any non-white president must be decried as not a citizen.
The second layer implies that an American woman can't have her baby abroad. While not directly related to Obama (he was born in Hawaii, after all), the implications of this are staggering for other US citizens. They're trying to argue that if a US citizen gave birth to a child outside the United States to a non-native father, he would be ineligible to be a natural born citizen. Which is ridiculous. Although, the law is rather unclear on this regard, but that doesn't stop Orly Taitz from spreading legal disinformation about it.