Racism is judging someone negatively based on their race, which Trump did pretty blatantly here and has done consistently throughout his campaign/life. He's said various derogatory things about Mexicans, and now fears retribution from someone of Mexican descent; which is something that many of his followers can relate to.
It's important for his supporters to explain this away because clear pandering to racists paints them negatively; they're well aware that it's socially unacceptable to be "racist".
To be clear, there's a spectrum of racism from somewhat insensitive, to Paula Dean, to the Klan. Most racists tend to draw the line somewhere above themselves, ie they're not racist because they're not Klan members; in the same way that Paula denies racism.