From crunchbase (https://www.crunchbase.com/organization/benchmark/investment...) it looks like Benchmark made three investments in Uber, the last of which was in 2013. The funds that VCs raise normally only operate for a specific term of eg 5 years, so now is the kind of time they will want to get out.

Incidentally from what I can make of their investments in Uber on crunchbase, they need Uber's market cap to be $31bn just to break even on their investment (13% of the business for $411m). That's probably what they're fighting for.