It appears to be because the hyphen implies a discretionary break after it. Since the line is overfull no matter what, the break is taken, and leaves a lone space token that winds up on a line of its own.
If you insert
\showboxbreadth=999
\showboxdepth=999
\showlists
after the long line (with a blank line in between), you will see this in the log file:
### vertical mode entered at line 0
### current page:
\glue(\topskip) 3.16669
\hbox(6.83331+0.0)x469.75499
.\hbox(0.0+0.0)x20.0
.\tenrm T
.\tenrm t
.\tenrm t
…
.\tenrm t
.\tenrm t
.\tenrm -
.\discretionary
.\glue(\rightskip) 0.0
.\rule(*+*)x5.0
\penalty 400
\glue(\baselineskip) 12.0
\hbox(0.0+0.0)x469.75499
.\glue(\rightskip) 0.0
total height 22.0
goal height 643.20255
prevdepth 0.0, prevgraf 2 lines
Interestingly, there is no \parfillskip
there, so maybe my theory is lacking some piece of the puzzle. But you do get the same result if the dash is replaced by \discretionary{}{}{}
.