One of the "interesting" things about the new strain is that generally there is genetic drift happening all the time with the virus and you can trace it internationally into a sort of 'family tree'. This can be used to work out how it is spreading geographically, its like a fingerprint that changes as it spreads.

e.g. the tree diagrams in this tweet: https://twitter.com/firefoxx66/status/1340359989395861506

The new strain is notable because it has no obvious ancestors - it has about 14 or so changes that can't be traced backwards in the tree. This points to it arising within one immunocompromised individual.

See https://virological.org/t/preliminary-genomic-characterisati...

"The accrual of 14 lineage-specific amino acid replacements prior to its detection is, to date, unprecedented in the global virus genomic data for the COVID-19 pandemic."

"These considerations lead us to hypothesise that the unusual genetic divergence of lineage B.1.1.7 may have resulted, at least in part, from virus evolution with a chronically-infected individual. Although such infections are rare, and onward transmission from them presumably even rarer, they are not improbable given the ongoing large number of new infections."

In all of this, its worth remembering we've never tracked an epidemic in this much detail before, so perhaps this sort of thing is just par for the course.