95 300gt starts when connecting the battery
 

so I have a 95 3000gt I purchased from my brother, he drove it every day for a couple years and had no issues. I parked it in my garage for the winter, in the spring I pulled it out, the battery was dead, I put battery charger on it and it instantly started turning over and started... the ignition was off and key laying in the console. only way I could get it to shut off was disconnect the battery. ive charged the battery while disconnected but as soon as I hook it up car starts. I have no idea where to even start, any help would be much appreciated