Using 'mas' as 'porque' expresses a sort of excuse for the sentence which precedes it, an assumption; usually followed by the verbs ‘dever’ and ‘poder’ . For instance:
He has not talked to me; he might not have seen me.
Ele não falou comigo; mas não deve ter me visto.
If I had used ‘porque’ it would sound as if I was sure he had not seen me. Although, I am not sure all the native speakers would tell the difference. Normally I’d say “Ele não me viu, deve estar cansado’ in this particular case.
To me, this is the only case they have similar meanings. I have double checked that with the Houaiss dictionary of Portuguese language (2009), in which this is the least common acceptation for ‘mas’. For other cases ‘porque’ is related to because, for or as.