wrote:

When I made a connection af Frankfurt from a Lufthansa filght at one of
the A gates to a United flight at one of the C gates, I had to go
through an air side security check point. Or, at least, it never felt
Why did they do this?

Additional check for passengers to the U.S. Even when you start your
journey to the U.S. at Frankfurt (and other european airports as well,
as far as I know) you are checked twice.

or do passengers arriving on LH
present more of a risk?

In this case it has nothing to do with from where or on what you are
arriving.
But additional checks for transiting passengers arriving from outside
Europe seem to be common on european Airports nowadays as well. Last
November I was checked airside after immigration at Amsterdam coming
from Seoul connecting to Germany.