It's called Bitcoin Credit. It allows you to spend an amount you don't have up to (the number of years that address has been in existence  X upper most amount of BTC it once had) up to a maximum of 1 BTC total .
Examples:
 I once had 0.25 BTC in an address that has existed for 1/2 year. I'd be able to spend from that same address on Bitcoin Credit of 0.125 BTC
 I once had 2.45 BTC in an address tha has existed for 3 days. I'd be able to spend from that same address on Bitcoin Credit 0.02013 BTC
 I once had 1.24 BTC in an address that has existed for 3.4 years. I'd be able to spend from that same address on Bitcoin Credit 1 BTC

You have 1 year to pay it back + a fixed 20%, or the bitcoin Men In Black, or Putin, will come for you.   The 20%, goes to the core developers to spend on upcoming features. Or beer. 

I'm kidding, of course.
It's just a double spend or an attempt at a double spend. Most times this is not someone trying to spend coins twice in a malicious way. Most times, but not always there are many reasons it could be..but most times it is someone who submitted a transaction with too low of fee and re-submitted it with a higher fee. Until his 1st low fee trans is dropped, it would show a negative balance. One one would ever confirm. But it takes 3-7 days or more depending on the software that is submitted it, to be dropped.