The more I think of it, the more I think that phone calls with Trump surrogates were tapped. I think that there were/are investigations into Russian hacking and that calls coming from Russia are being screened, it should be abundantly clear that anybody having conversations with foreign actors with bad intentions have chance of being tapped and taped, so the calls were likely screened not because of anybody on Trump's side but people on the other end of the line. They are tapping the Russians but when they are calling an American they can get caught up in the giant fish net. My working theory is that when the first findings of this were brought to Trump's attention he either made the assumption that they were brought on by Obama or is using that line of offensive attack as cover. If it's the former, he lacks an understanding of intelligence gathering as it pertains to the laws of the country. If it is the latter, it is to try to delegitimize the process and throw up a giant smoke screen since he feels the walls of justice closing in on him.
Either that or he just made the entire thing up which sadly might be the most likely scenario.