In the right hand tab, under "modules", you can disable subsystems. Basically, your power plant will put out a certain amount of power, which is then used by your modules in one of two modes: deployed and undeployed. Deployed is when you have your weapons out, and generally requires more power. You can see your current power supply and requirements in a station outfitting shop.
In your case, your undeployed requirements are clearly within your power plant's output, but your deployed requirements are above. Because of this, when you deployed hardpoints, the ship chose some modules to switch off. I'm not sure of the logic it uses to pick which, but it's usually Bad.
If you don't want Bad, there are three options. 1) Fix your loadout so that your deployed power requirement is within your power plant's output. 2) Manually disable enough modules, such as your fuel scoop, that you don't want to use when you're expecting a fight (highlight the module in the right-hand tab and hit select). 3) Give your ship some assistance choosing modules to automatically disable by setting priorities. For example, you really don't want to lose shields, as once they go down they take ages to regenerate, so this module should be priority 1. On the other hand, since you probably don't expect to be scooping fuel while your weapons are out, you can set the fuel scoop to priority 2, and it'll be disabled automatically when you deploy hardpoints. Change priorities by highlighting the module and hitting right/left.
Up to a point, you can avoid this by buying better gear, but when running A class modules, there are some loadouts that just can't handle deployed hard points without using the modules tab, so it's worth getting familiar with it.