Here's a theory.
I may be totally off - but this may play into Civil War. What if Hawkeye's married life a lie? What if it is part of a SHIELD brainwashing event OR possibly a Skrull infiltration?
When I saw Avengers: Age of Ultron and saw that Hawkeye had a wife (and kids?!) it always threw me off. It totally threw out the flirting between Hawekeye and Black Widow in the first movie, and made for the possibility of Mockingbird and Hawkeye a little more difficult.
First theory - SHIELD brainwashing. We've seen it done before with former SHIELD agents and with current agents as a form of therapy. Hawkeye gets a new life with wife and kids as a form of retirement.
Second theory - Skrull infiltration. This one is a little more out there and can lead to a few "wtf" moments. Either using Skrull (or other Skrull-type alien) infiltrating SHIELD via Hawkeye.
Once he discovers this - drama ensues, Hawkeye meltdown. He eventually runs into Mockingbird (after losing her current love interest on Agents of SHIELD - sorry, I like you - but you are deader than dead...) and things move forward.