On two occasions, I have heard this announcement "cabin crew, please take your seats for take off now" with a particular emphasis on the now. Once by the pilot himself and once by the senior cabin crew. Only then did the crew rush for their jump seats.
I am guessing that a plane in the queue in front (either for landing or take off) had to leave the queue for some reason with the cockpit receiving a late notification to speed up and take their space.
Normally, the "please take your seats" announcement is made well ahead of time (on the ground for take off anyway).
I still like the "welcome message on landing" announcement made by the cabin crew member once. "Well, I am sure you would have felt that we have landed at ………………. Welcome blah blah". It was a particularly hard landing (and I do with a very loud thud that shook every bone) by the pilot that day. Most of the passengers did get the sarcastic tone and laughed along with relief.