And she was already 'in trouble' with him as on the day he murdered the girls she was away in Grimsby spending time with her family. He didn't like her going out and drinking or being with her family so called her in the morning and she didn't pick up. Speculation on my part but probably because she knew he'd be pissed off and angry and ruin her day.
She phoned him back in the evening and he blew up at her because she hadn't called him back earlier and because she said she would be going out drinking with her family. He then put the 'phone down on her after a few minutes. She texted him saying 'don't make me feel bad because i'm with my family' and he didn't reply.
It was shorty after that he murdered the girls. The call ended at 6.23pm. The girls were last seen alive at 6.05pm leaving Hollys house. Given the girls route, and Huntleys initial statements, that he 'saw' the girls around 6.30pm passing by his house while he was outside washing his dog, it was around that time he lured the girls into the house. Jessicas mobile phone was switched off at 6.46, presumably by Huntley.
Huntleys own Mother speculated that he killed the girls because he was angry at Maxine Carr.
So Maxine gets home from being away, already frightened because she knows he's angry she didn't call him back quick enough the previous day and has been out drinking with her family when he said she shouldn't. And 2 girls are missing that she knows and he's telling her that because of a past false rape accusation (it wasn't false but he wouldn't say that), the Police will try and pin it on him and she needs to give him an alibi.
Like she said in court, she was scared and felt backed into a corner but she also didn't think at that time that he'd killed them.