Why is there a spark as the worker connects to the power lines?

The helicopter is at earth potential when it leaves the ground. When it comes close to the power lines, which are 500,000 Volt, the helicopter is still at earth potential. When the two come close enough together, the air between them will break down and equalize the potential between the helicopter and the wire.
The bigger the helicopter, the more charge it will hold, and the bigger the arc between the two. The arc is constant as the worker holds out the discharging wand and contacts the wire.
(This is different from static electricity, where you get just one strike which instantly equalizes the charge.) The reason is that the cable carries A/C (Alternating Current). As the potential changes, the helicopter is charging and discharging 50/60 times a second.