Some countries require visa applicants to show proof that they've already bought flight tickets etc. for the entire trip by the time they apply for a visa. I'm not sure whether Spain is one of them, but we have anecdotal evidence that some Schengen countries do -- or at least some consulates of some Schengen members do.
On the other hand the UK explicitly don't want to see flight tickets and paid-for hotel bookings. They recommend not to spend any money before you know you have a visa.
This means that if your mother is the tiniest bit unsure whether she will qualify for visas, applying to the UK first gives her maximal flexibility to adapt her travel plans to whether or not she can go there, when she later applies elsewhere.
In contrast, if she starts by applying to somewhere that requires all of the tickets to be present, she'll need to lock in her choice to go to the UK or not already at that time, before she knows if she can get a British visa.
If I remember correctly, the UK visa application asks you if you have ever been refused a visa to anywhere, whereas the harmonized Schengen application form doesn't. So if your mother happens to get a refusal from the UK first, she can get a reasonably fresh view of her application from Spain later on. But if she applies to Spain first and is refused there, she would have to disclose that something's up when she applies to the UK later, possibly resulting in tighter scrutiny of her circumstances.