With string[10:00:00], you are calling the string function on the input 10:00:00. With string (10:00:00) ," abc" , you are acually joinng (10:00:00) to "abc" and then stringing the results. You have to remember that execution is carried out from right to left.
q)(10:00:00) ," abc"
10:00:00
" "
"a"
"b"
"c"
q)string (10:00:00) ," abc"
"10:00:00"
," "
,"a"
,"b"
,"c"