I think it stems from the meeting when Michael asked Tessio & Clemenza to be patient. Clemenza asks Vito for permission to form his own family and leave the Corleones. This request showed that Clemenza didn't believe that Michael would be able to solidify the family. With a weakened Corleone family, Michael, Vito & Tom correctly assume that one or more of the rival families would reach out to either/both Clemenza & Tessio.

Tom felt it would be Clemenza based upon Clemenza's resistance to Michael's proposal. To a lesser degree, I think Tessio did a much better job of masking his emotions overall. Clemenza was more apt to "wear his heart on his sleeve" and revealed his true feelings in the meeting with Michael, Vito & Tom.