As other commenters have noted, it was an installation subsidy. Installation is a major cost of solar, other than just the cells themselves. That made it cheaper to move inventory out of the warehouse and into the field. With the subsidy gone, the inventory piles up in the warehouse and manufacturers get desparate to sell. So they drop their prices, but it's still more expensive to bring those panels online than it was when the subsidy was in force.

It's possible that the price will keep dropping until the panels are cheap enough to move at their current production capacity. But it's also possible that the lost profits make this level of production unsustainable, and solar panel costs climb back up as manufacturers shut down their no longer profitable lines and supply drops. It depends partly on what their margins are now, and what cost reduction room they have.