YoyoBoar Lin

Hello guys, I am a new Houdini user, and my english is limited, may I try to say my problem with pyro simulation by my limited english.
recently I did an explosion by pyro simulation, this simulated from 20 to 120 frame, the simulate smoke hang on in the air that looks like doesn't move forward, just stop in the air from 48 frame to 72 frame, then smoke start move again after 72 frame.
I try to figure out what reason makes smoke stop in air.
I thinking the problem could be collision object, every one have any idea?

thank you so much guys.

smoke simulation

smoke collision