Why did all my systems shut down when I deployed hardpoints?

After earning some credits trading, I traveled to the station where I had a basic Eagle, swapped to the Eagle, spent some credits on upgrades by swapping out the two default pulse lasers for 3 of the best weapons I could buy, adding in a frame shift drive interdictor, a warrant scanner and maybe something else, so that I was well-equipped for some bounty hunting. I set the fire groups up in a reasonable fashion.

Eager to try out my new fighter, I traveled a bit slowly to another station until somebody else FSD interdicted me, I got ready to fight by targetting them, throttling up to 50%, and deploying hardpoints. When I deployed hardpoints, though, I got a dire warning about power usage and everything shut down; no weapons, no thrusters, no nothing! After a few moments of panic, I was able to retract my hardpoints and run away.

Why did deploying hardpoints use up all of my energy? How do I prevent that from happening again?