Strange floating base NPN behavior
You have wired these two transistors in what is very close to a &quot;Darlington&quot; configuration. (A couple of comments correctly pointed out that this is not strictly Darlington, but it is close.)
In this mode you have made a VERY high-gain amplifier since the gain is the product of the individual transistors.  In the case of the UNR4216 you have this:

hFE is the forward current gain of the transistor.  So you have a gain with the pair of these in the range of:
160 * 160 -&gt; 460 * 460 or about 25k - 200K
So any input on &quot;B&quot; is going to be amplified significantly, even normally minor things like leakage current.
When you ground (i.e. hook &quot;B&quot; to &quot;G&quot;) you are forcing the transistors to the OFF state.  They are effectively open circuits and your LED is OFF.
When you &quot;float&quot; the &quot;B&quot; pin, you basically have an antenna that picks up whatever is around it and does what it does.  In this case it partially turns on Q2 and some current flows and your LED flickers as the picked-up signal on &quot;B&quot; fluctuates.
If you were to tie your &quot;B&quot; to the 5V source, through a currently limiting resistor, say 10K Ohm, then you will fully turn both Q1 and Q2 ON and the LED will turn on fully.
BTW, you should also put a current limiting resistor on the collector of Q1 to avoid damaging it or Q2.  See the datasheet on the UNR4216 for maximum current ratings for Ibe (Base-Emitter Current) and other relevant parameters.