From Orbcomm OG2 Mission 1 Update (June 20, 2014):
During today’s countdown, engineers noticed fluctuations in pressure
on the Falcon 9 vehicle’s second stage and are taking additional time
to evaluate. The next available launch opportunity is tomorrow
evening, Saturday, June 21st. Additional updates will be posted as
schedules are confirmed.
And Spaceflight 101 put it like this:
Late in the countdown, a pressure drop on the rocket's second stage
was noticed during regular pressure checks leading to a scrub of the
launch when the team ran out of launch window time.
As also described in Spaceflight Now article: Delay-stricken SpaceX launch scrubbed by technical issue, this has however already happened earlier in the countdown, shortly before beginning the terminal countdown for liftoff at the beginning of the launch window and the countdown clock was recycled with the next launch attempt at the end of the roughly one hour long launch window. Problems were not resolved before the last station polling for the day, and the launch was scrubbed with the next launch opportunity one day later, June 21.
Weekend launch opportunities on June 21 - June 22 were also scrubbed. Saturday one for the weather, and Sunday one due to problems with thrust vector control actuator on the first stage, according to Gwynne Shotwell, SpaceX president and COO, mentioning this during John Batchelor Show / The Space Show Special (roughly 3 minutes into the recording).
According to Orbcomm OG2 Mission 1 Update (June 23, 2014):
SpaceX is taking a closer look at a potential issue identified while
conducting pre-flight checkouts during yesterday's [June 22] countdown. SpaceX
will stand down Tuesday, June 24 while our engineering teams evaluate
further, which will also allow the Range to move forward with
previously scheduled maintenance. We are currently targeting the first
week of July and will work with the Range to confirm the next
available launch opportunities.
