Either they feel that it is time for president kimballa or perhaps she will not take the position which would install Pelosi or it might even make it's way to president blinken.
Next in line for President
No. Office Incumbent
1 Vice President Kamala Harris
2 Speaker of the House of Representatives Nancy Pelosi
3 President pro tempore of the Senate Patrick Leahy
4 Secretary of State Antony Blinken
Those people don’t cascade into positions. The only way a person 3rd or 4th on the list takes the seat is if multiple people get taken out simultaneously.