John Gill Commentary 2 Samuel 12:21

"Then said his servants unto him, What thing is this that thou hast done? thou didst fast and weep for the child, while it was alive; but when the child was dead, thou didst rise and eat bread." — 2 Samuel 12:21 (ASV)

Then said his servants unto him, what thing [is] this that
you have done ?
&c.] Or what is the reason of such conduct and behaviour? they knew what was done, but they did not know the meaning of it, which is what they inquired after:

you did fast and weep for the child, [while it was] alive ;
prayed with fasting and weeping for it, that it might live and not die:

but when the child was dead you did rise and eat bread ;
and appeared cheerful; this seemed strange to them, when they expected his sorrow would be increased.