I didn't really find Topsy's response to be strange. Whenever I watched it, not at one time did I ever think "That was an odd response for him". Topsy basically still acted like himself, blaming Littlefoot for what happened. He may have realized that he didn't have a legitimate reason to blame him, no proof, thus he stammered when Grandpa Longneck questioned him on why he thought it was his grandson's fault.
I think by this time, Topsy respected Grandpa Longneck, even if he did not show it, and may have felt guilty, even just slightly, over his brash blame game on Littlefoot. The way he reacted, the way he spoke, suggests to me guilt, perhaps even shame, may have been involved. Even his facial expression leads me to conclude this.
I may decide to rewatch the scene sometime to make sure I didn't overlook any detail.