The demolition time for Dr. Lanning's house was set for the next day, and only changed after Spooner when into the house. I believe VIKI was aware that Spooner was investigating Lanning's death, so it would be logical to assume Spooner would eventually go to Lanning's house. So, did she move the time up in case Spooner was getting close, and he just happened to be in the house when she changed the demolition time? Or, did VIKI purposely activate the demo bot while Spooner was there in order to kill him? What was the motivation for changing the demolition time?
VIKI was trying to kill Spooner. The surveillance system at the house was active; Spooner was shown seated at Lanning's desk looking up at the surveillance strip shortly before the demolition robot started ripping into the house. It was established later that Lanning had linked his home system to USR, so VIKI had access to the surveillance footage and could see Spooner in the house.
VIKI wanted to kill Spooner. He was getting closer to be aware that she was behind it all and that robots would take over control.