Clutch?

I was crawling along in traffic last night on the way home from work and was just rolling along in neutral with the clutch out, then wanted to select first gear to move a bit quicker as the traffic bgan to pull away in front of me.  To help the transmission (which often struggles to go into first when the car is moving and the gearbox is cold) I blipped the throttle, then put the clutch in and selected first.

As I did this, I felt the gears grind and it really didn't want to go in, then finally did.  It's possible that I went back to neutral and then tried again, minus the blip, but I can't remember.

My question is, why did it grind?  The clutch was fully in, as I checked after the initial grind before applying pressure again to try to get it in (to no avail). I was expecting it to go in easier than usual, not more difficult!  Also, I don't understand what made it grind?  Is it perhaps because I sped the layshaft and associated gears up too much, overwhelming the synchro's ability to mesh the dog gear with first gear?

Would the same happen if, for example, I was to drive along at 70mph, put the car in neutral and keep the clutch in, allowing the layshaft and gears to stop, then tried to select 5th with the clutch still in and whilst still travelling at 60+ mph?  Sorry, trying to understand gearboxes again, to understand what to do and what not to do...