darse cuenta = to notice (exactly: to give oneself the bill) derrumbarse = to break down

As also in English when the actual process of something is in the center of attention the continuous form is used also in Spanish as you can see:

Estaba bebiendo una coca cola, cuando la casa se derrumbó. = I was drinking a coke when the house broke down. Le estaba ayudando, cuando me di cuenta que no se le podía ayudar. = I was just helping him when I noticed that he can't be helped.