Venture funds typically lose money in their first few years, the management fee is paid up front, early markdowns happen on companies that fail, and exits are still years away. The cumulative return chart starts negative and curves up as later exits materialize, producing the characteristic J shape.
The shape is structural, not pathological. A fund that doesn't show the J early is suspiciously aggressive on marks; a fund that never recovers from the J never returns capital. Sophisticated LPs underwrite the trough depth and recovery rate as much as the headline return target.