Well the FBI wire tapped Manafort because of supposed ties to Russia, not Trump himself. 
[quote]A secret order authorized by the court that handles the Foreign Intelligence Surveillance Act (FISA) began after Manafort became the subject of an FBI investigation that began in 2014. It centered on work done by a group of Washington consulting firms for Ukraine's former ruling party, the sources told CNN.[/quote]

[quote]Sources say the second warrant was part of the FBI's efforts to investigate ties between Trump campaign associates and suspected Russian operatives. Such warrants require the approval of top Justice Department and FBI officials, and the FBI must provide the court with information showing suspicion that the subject of the warrant may be acting as an agent of a foreign power.[/quote] 

But Obama himself didn't wiretapped Trump, which is what Trump claimed.