While we are free to speculate, we should realize that we dont have any of the facts. This may likely be something minor that Trump is using to augment and promote himself by getting people upset about the system.
On the other hand, for those that are saying that this must be a very legitimate and serious concern if the FBI went through the hoops and was able to get a warrant. In the past, I would have thought the same.
However, the Mueller investigation, has completely changed my mind. No longer are serious investigations necessarily legitimate, even if they are conducted by the highest levels of authority.
We now know that the there was no legitimate basis for this investigation, the people that authorized it knew it as well. Yet, it held us all captive and at the time made us all think that Trump might be guilty of treason. The fact that other unrelated carious white collar crimes were uncovered, does not justify any of this.
Therefore, no longer is a warrant or the fact that there is an investigation any indication that there is a legitimate issue here.