How was the Resistance exposed?

Question

At the start of The Last Jedi the Resistance is in full retreat. From the opening crawl:

The FIRST ORDER reigns. Having decimated the peaceful Republic, Supreme Leader Snoke now deploys his merciless legions to seize military control of the galaxy.

Only General Leia Organa’s band of RESISTANCE fighters stand against the rising tyranny, certain that Jedi Master Luke Skywalker will return and restore a spark of hope to the fight.

But the Resistance has been exposed. As the First Order speeds toward the rebel base, the brave heroes mount a desperate escape...."

How was the Resistance exposed exactly? What caused them to evacuate?

Answer 1

They were exposed because they sent out a recon flight to find Starkiller base...

“The scan data from Captain Snap Wexley’s reconnaissance flight confirms everything Finn has told us,” Poe announced to the group.
The Force Awakens novelization by Alan Dean Foster, Chapter XIV

... and that flight was tracked back to their home base:

Hux was clearly gratified to be the bearer of good news. “We were able to track their reconnaissance ship back to the Ileenium system. We are coordinating with our own reconnaissance craft in the area in order to lock down the specific location of their base.”
Snoke replied with cold satisfaction. “We do not need it. Prepare the weapon. Destroy their system.”
The Force Awakens novelization by Alan Dean Foster, Chapter XV

and

The general halted before the dais and bowed. “We were able to track their reconnaissance ship back to the Ileenium system. We are coordinating with our own spy vessels to lock down the specific location of their base.”
“We do not need it. Prepare the weapon. Destroy their system,” the Supreme Leader said.
The Force Awakens Junior Novelization by Michael Kogge, Ch 16

Answer 2

The First Order found out their location before the destruction of their Starkiller Base in the previous film. Having failed to destroy it with their super-weapon, they resort to more conventional means; sending a big fleet of ships to blow it up from orbit.

GENERAL HUX: We have their location. We tracked their reconnaissance ship to the Ileenium system.

SNOKE: Good. Then we will crush them once and for all. Prepare the weapon.

_{^{Star Wars: The Force Awakens}}

As a nice little easter egg, we finally know how the First Order was able to track the Resistance's ship back to its base, presumably by using the hyperspace tracking technology that appears in The Last Jedi.