Why was he caught?
Because he loved animals
Bjornsdottir took notice of the attention the couple routinely paid to an abandoned, tiger-striped cat in the neighborhood, and eventually grew close (or at least closer than most) to Greig [Whitey's girlfriend]. It was that bond, over the cat, that ultimately led to the fugitives' capture, according to the paper.
she saw a CNN report on the FBI’s latest effort to track the 82-year-old Bulger and his 60-year-old girlfriend, Catherine Greig. Bjornsdottir recognized them immediately as the Gaskos, her former neighbors—Tiger’s benefactors—an ocean away on Third Street.
More at: http://slatest.slate.com/posts/2011/10/10/james_white_bulger_former_miss_icleand_anna_bjornsdottir_was_fbi.html