Sterling has fallen because of Brexit. This increases the cost of importing parts, etc. from foreign suppliers.

Brexit was also expected to have already occurred so customers may have taken a cautious approach regarding orders that may have spanned Brexit occurring.

I guess there is also the issue of maintenance and sourcing parts from them during the life-time of the busses.

It is not unreasonable to accept that Brexit may have caused customers to conclude that it was safer to pick other suppliers for the time being.