Union reps and branches of both Aslef and RMT are organising to resist LU's imposition of emergency timetables. The company wants to ramp up the train service, but won't provide drivers with adequate PPE, thereby exposing them to greater risk.
As and when lockdown measures ease, the service will need to be phased back up. That must be done in consultation with union reps and in a way the ensures passenger and staff safety. Imposing new timetables that drivers' reps have already rejected is a reckless move from LU, and with passenger levels down by about 95%, is not operationally required. The move may also impact station staff, by requiring some stations to open earlier, thereby requiring adjusted duty times for station workers.
Some branches have called for formal disputes and ballots. This is an important step, to send a signal of intent to the company. But in reality the length of time these procedures take (notification of a formal dispute; notification of the ballot; conducting the ballot; notification of any action...) mean that LU may well have imposed the timetables anyway before the official procedures have really gotten going. That's why reps are right to remind workers of their right to withdraw from a task or situation they feel places them in "serious and imminent danger".