Sparking the wildcat walkouts was management's response to drivers' refusal to cross picket lines at other depots. The postal workers' union, the CWU, have called “rolling strike action,” where different parts of the company stop work at different times. The idea being (I assume) to cause maximum disruption to the company without going “all-out”. The second phase of this started on Tuesday 31st, when mail centres and cash handling stopped from 3AM.
At one office in the West of Scotland a driver refused to cross the picket line at Springburn on arrival there that day. On return, management said they would be docking an hour's wages for that and workers responded.

“We went out, he was told he wouldn't get it docked by our manager, we went in, he was then told by some high-up manager he would be docked, we went back out. Finally he persuaded us to go back in because he didn't want us to lose out for him.”

Similar situations seem to have been replicated across the West of Scotland. In Paisley, a larger office with more drivers, the workers stayed out longer and held a mass meeting Tuesday afternoon. The “rolling” structure of the planned strikes and apparent orders from on high to punish workers who refuse to scab by crossing their colleagues' picket lines means the situation is likely to further escalate, both geographically and tactically.