Teams are being very cautious at the World Cup this year to make sure they don't accidently put themselves in a position where they could end up allowing a very easy goal to their opponents.
After big goals, such as the Cristiano Ronaldo free kick that leveled Portugal with Spain at 3-3 or the Harry Kane header that lifted England over Tunisia, teams have been good to break into celebrations that involve the whole squad, except for one player.
In the cases of both Portugal and England, one player–Jose Fonte for Portugal, Kieran Trippier for England–made sure to stay on the pitch and not leave the field of play to prevent the opposition from kicking off, or at least cover themselves in case the laws of the game are interpreted in a way that could allow that. According to FIFA's Laws of the Game, all of the players except the one taking the kickoff must be in their own half of the field of play. If interpreted that players off the playing field entirely are not subjected to the same regulations as those on it, then the opposing team could theoretically kick off and resume play during a full-team celebration, while the goalkeeper stands alone against 10 men. The opposition could also restart as soon as the scoring team all goes back to its side of the field, even in celebration, as long as it has the referee's signal.
So, as England and Portugal have evidently elected to understand it, as long as one of the 10 outfield players remains on the pitch in the opposing team's half, there cannot be a kickoff to immediately resume the game. But, if all 10 outfield players leave the field, or the players who remain on go to their half of the field, the game can be resumed. It may be paranoid or even unnecessary, but they're covering themselves nonetheless.
Considering this has already come into play twice in the first few days of the tournament, expect to see it come up again.