I've seen this problem reported since SourceTree 2.7 and I can't believe it's still not fixed in 2+ years, therefore I hope I'm doing something wrong.
Bug Description:
SourceTree consumes all of MacBook's CPU while doing exactly nothing.
Repro Steps:
1) Open SourceTree
2) Open a Repository (BigBucket connected - I don't know if this matters?)
3) Do exactly nothing
4) Watch the CPU consumption to grow and grow above >200% - untilt the MacBook's fan goes crazy