My mental model of Lucius says he believes...

• Harry is Voldemort (Who is still strong in some ways but weak in others)

• Harrymort has been trying to split Draco from his father.

• Harrymort has been using a new ideology to recruit Draco.

• This new ideology is incompatible with the previous ideology.

• Harrymort has no use of his old allies as he hasn't let them in on anything and he's using an incompatible ideology.

• Harrymort's gathering of power and allies/minions (eg Hogwarts students) will take at least a decade to come to fruition.

• Harrymort is in a weak position right now and in danger of losing a key piece. (Hermoine would be his Bellatrix.)

• Harrymort seems to be on Dumbledore's side in the court room and ideologically.

So, assuming Lucius isn't just acting on rage, he's decided to align himself against Harrymort and with whoever it is that's attacking him. Perhaps he's hoping that he can destroy Harrymort soon enough before he becomes invincible withe the passage of years. Perhaps he hopes by aligning the House of Malfoy against Harrymort publicly, whoever attacked his son won't target him again and risk losing the Malfoy's support. Perhaps he thinks Harry is strongly allied enough to Dumbledore to lie to Lucius. Although I'm more of the mind that he's acting on blind revenge, or at least it's clouding his vision enough to make poor choices at the moment.