Why does the Tx pin of an USB to TTL adapter provide current?
The USB&lt;->TTL_serial adapter is being powered from USB.
One of its main jobs is to drive the TX signal line to whatever state is appropriate for whatever communication is taking place.
If there's no comms happening, the default state for a TTL UART TX line is high (so 3.3V or 5V or whatever the device's supply is).
As a result, you're feeding this 'high' into your other circuit (ATmega, adruino, whatever).
So now what does this other circuit do with this 'high'?
Well most microcontrollery things (and many others too) will have some sort of diode arrangement between their I/O pins and their supply lines - either deliberately as ESD protection, or simply as an unavoidable result of the manufacturing process - something like this:  







simulate this circuit &ndash; Schematic created using CircuitLab

So as you can see, if your USB&lt;->TTL_Serial adapter is trying to do its job and apply a 'high' to the micro's I/O pin, due to the micro's internal diodes it will end up trying to supply power to the whole circuit. Some adapters may be more or less tolerant of this state of affairs than others...